ext4: implement swap_activate aops using iomap

After moving ext4's bmap to iomap interface, swapon functionality
on files created using fallocate (which creates unwritten extents) are
failing. This is since iomap_bmap interface returns 0 for unwritten
extents and thus generic_swapfile_activate considers this as holes
and hence bail out with below kernel msg :-

[340.915835] swapon: swapfile has holes

To fix this we need to implement ->swap_activate aops in ext4
which will use ext4_iomap_report_ops. Since we only need to return
the list of extents so ext4_iomap_report_ops should be enough.

Cc: stable@kernel.org
Reported-by: Yuxuan Shui <yshuiv7@gmail.com>
Fixes: ac58e4fb03 ("ext4: move ext4 bmap to use iomap infrastructure")
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200904091653.1014334-1-riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -3601,6 +3601,13 @@ static int ext4_set_page_dirty(struct page *page)
 	return __set_page_dirty_buffers(page);
 }
 
+static int ext4_iomap_swap_activate(struct swap_info_struct *sis,
+				    struct file *file, sector_t *span)
+{
+	return iomap_swapfile_activate(sis, file, span,
+				       &ext4_iomap_report_ops);
+}
+
 static const struct address_space_operations ext4_aops = {
 	.readpage		= ext4_readpage,
 	.readahead		= ext4_readahead,
@@ -3616,6 +3623,7 @@ static const struct address_space_operations ext4_aops = {
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate  = block_is_partially_uptodate,
 	.error_remove_page	= generic_error_remove_page,
+	.swap_activate		= ext4_iomap_swap_activate,
 };
 
 static const struct address_space_operations ext4_journalled_aops = {
@@ -3632,6 +3640,7 @@ static const struct address_space_operations ext4_journalled_aops = {
 	.direct_IO		= noop_direct_IO,
 	.is_partially_uptodate  = block_is_partially_uptodate,
 	.error_remove_page	= generic_error_remove_page,
+	.swap_activate		= ext4_iomap_swap_activate,
 };
 
 static const struct address_space_operations ext4_da_aops = {
@@ -3649,6 +3658,7 @@ static const struct address_space_operations ext4_da_aops = {
 	.migratepage		= buffer_migrate_page,
 	.is_partially_uptodate  = block_is_partially_uptodate,
 	.error_remove_page	= generic_error_remove_page,
+	.swap_activate		= ext4_iomap_swap_activate,
 };
 
 static const struct address_space_operations ext4_dax_aops = {
@@ -3657,6 +3667,7 @@ static const struct address_space_operations ext4_dax_aops = {
 	.set_page_dirty		= noop_set_page_dirty,
 	.bmap			= ext4_bmap,
 	.invalidatepage		= noop_invalidatepage,
+	.swap_activate		= ext4_iomap_swap_activate,
 };
 
 void ext4_set_aops(struct inode *inode)