[GFS2] gfs2_alloc_required performance

This is a small I/O performance enhancement to gfs2.  (Actually, it is a rework of
an earlier version I got wrong).  The idea here is to check if the write extends
past the last block in the file.  If so, the function can save itself a lot of
time and trouble because it knows an allocate will be required.  Benchmarks like
iozone should see better performance.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/bmap.c

@@ -1224,6 +1224,11 @@ int gfs2_write_alloc_required(struct gfs2_inode *ip, u64 offset,
 		unsigned int shift = sdp->sd_sb.sb_bsize_shift;
 		lblock = offset >> shift;
 		lblock_stop = (offset + len + sdp->sd_sb.sb_bsize - 1) >> shift;
+		if (lblock_stop > ip->i_di.di_blocks) { /* writing past the
+							   last block */
+			*alloc_required = 1;
+			return 0;
+		}
 	}
 
 	for (; lblock < lblock_stop; lblock += extlen) {