printk: use kvmalloc instead of kmalloc for devkmsg_user

Size of struct devkmsg_user increased to 16784 by commit 896fbe20b4
("printk: use the lockless ringbuffer") so order3(32kb) is needed for
kmalloc. Under stress conditions the kernel may temporary fail to
allocate 32k with kmalloc. Use kvmalloc instead of kmalloc to aviod
this issue.

qseecomd invoked oom-killer: gfp_mask=0x40cc0(GFP_KERNEL|__GFP_COMP), order=3, oom_score_adj=-1000
Call trace:
 dump_backtrace+0x0/0x34c
 dump_stack_lvl+0xd4/0x16c
 dump_header+0x5c/0x338
 out_of_memory+0x374/0x4cc
 __alloc_pages_slowpath+0xbc8/0x1130
 __alloc_pages_nodemask+0x170/0x1b0
 kmalloc_order+0x5c/0x24c
 devkmsg_open+0x1f4/0x558
 memory_open+0x94/0xf0
 chrdev_open+0x288/0x3dc
 do_dentry_open+0x2b4/0x618
 path_openat+0xce4/0xfa8
 do_filp_open+0xb0/0x164
 do_sys_openat2+0xa8/0x264
 __arm64_sys_openat+0x70/0xa0
 el0_svc_common+0xc4/0x270
 el0_svc+0x34/0x9c
 el0_sync_handler+0x88/0xf0
 el0_sync+0x1bc/0x200

 DMA32: 4521*4kB (UMEC) 1377*8kB (UMECH) 73*16kB (UM) 0*32kB 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 30268kB
 Normal: 2490*4kB (UMEH) 277*8kB (UMH) 27*16kB (UH) 1*32kB (H) 0*64kB 0*128kB 0*256kB 0*512kB 0*1024kB 0*2048kB 0*4096kB = 12640kB

Signed-off-by: Yong-Taek Lee <ytk.lee@samsung.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20210830071701epcms1p70f72ae10940bc407a3c33746d20da771@epcms1p7

kernel/printk/printk.c

@@ -847,7 +847,7 @@ static int devkmsg_open(struct inode *inode, struct file *file)
 			return err;
 	}
 
-	user = kmalloc(sizeof(struct devkmsg_user), GFP_KERNEL);
+	user = kvmalloc(sizeof(struct devkmsg_user), GFP_KERNEL);
 	if (!user)
 		return -ENOMEM;
 
@@ -875,7 +875,7 @@ static int devkmsg_release(struct inode *inode, struct file *file)
 	ratelimit_state_exit(&user->rs);
 
 	mutex_destroy(&user->lock);
-	kfree(user);
+	kvfree(user);
 	return 0;
 }