ext4: do not try to set xattr into ea_inode if value is empty

Syzbot report a warning that ext4 may create an empty ea_inode if set
an empty extent attribute to a file on the file system which is no free
blocks left.

WARNING: CPU: 6 PID: 10667 at fs/ext4/xattr.c:1640 ext4_xattr_set_entry+0x10f8/0x1114 fs/ext4/xattr.c:1640
...
Call trace:
ext4_xattr_set_entry+0x10f8/0x1114 fs/ext4/xattr.c:1640
ext4_xattr_block_set+0x1d0/0x1b1c fs/ext4/xattr.c:1942
ext4_xattr_set_handle+0x8a0/0xf1c fs/ext4/xattr.c:2390
ext4_xattr_set+0x120/0x1f0 fs/ext4/xattr.c:2491
ext4_xattr_trusted_set+0x48/0x5c fs/ext4/xattr_trusted.c:37
__vfs_setxattr+0x208/0x23c fs/xattr.c:177
...

Now, ext4 try to store extent attribute into an external inode if
ext4_xattr_block_set() return -ENOSPC, but for the case of store an
empty extent attribute, store the extent entry into the extent
attribute block is enough. A simple reproduce below.

fallocate test.img -l 1M
mkfs.ext4 -F -b 2048 -O ea_inode test.img
mount test.img /mnt
dd if=/dev/zero of=/mnt/foo bs=2048 count=500
setfattr -n "user.test" /mnt/foo

Reported-by: [e-mail]
Fixes: 9c6e7853c531 ("ext4: reserve space for xattr entries/names")
Cc: [e-mail]
Signed-off-by: zhangyi (F) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Theodore Ts'o <[e-mail]>

fs/ext4/xattr.c

···
 				 * external inode if possible.
 				 */
 				if (ext4_has_feature_ea_inode(inode->i_sb) &&
-				    !i.in_inode) {
+				    i.value_len && !i.in_inode) {
 					i.in_inode = 1;
 					goto retry_inode;
 				}