ext4: check fast symlink for ea_inode correctly

The check for a fast symlink in the presence of only an
external xattr inode is incorrect.  If a fast symlink does
not have an xattr block (i_file_acl == 0), but does have
an external xattr inode that increases inode i_blocks, then
the check for a fast symlink will incorrectly fail and
__ext4_iget()->ext4_ind_check_inode() will report the inode
is corrupt when it "validates" i_data[] on the next read:

    # ln -s foo /mnt/tmp/bar
    # setfattr -h -n trusted.test \
               -v "$(yes | head -n 4000)" /mnt/tmp/bar
    # umount /mnt/tmp
    # mount /mnt/tmp
    # ls -l /mnt/tmp
    ls: cannot access '/mnt/tmp/bar': Structure needs cleaning
    total 4
     ? l?????????? ? ?    ?        ?            ? bar
    # dmesg | tail -1
    EXT4-fs error (device dm-8): __ext4_iget:5098:
        inode #24578: block 7303014: comm ls: invalid block

(note that "block 7303014" = 0x6f6f66 = "foo" in LE order).

ext4_inode_is_fast_symlink() should check the superblock
EXT4_FEATURE_INCOMPAT_EA_INODE feature flag, not the inode
EXT4_EA_INODE_FL, since the latter is only set on the xattr
inode itself, and not on the inode that uses this xattr.

Cc: [email protected]
Fixes: fc82228 ("ext4: support fast symlinks from ext3 file systems")
Signed-off-by: Andreas Dilger <[email protected]>
Reviewed-by: Li Dongyang <[email protected]>
Reviewed-by: Alex Zhuravlev <[email protected]>
Reviewed-by: Oleg Drokin <[email protected]>
Reviewed-on: https://review.whamcloud.com/59879
Lustre-bug-id: https://jira.whamcloud.com/browse/LU-19121
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

Author: adilger

fs/ext4/inode.c

@@ -146,7 +146,7 @@ static inline int ext4_begin_ordered_truncate(struct inode *inode,
  */
 int ext4_inode_is_fast_symlink(struct inode *inode)
 {
-	if (!(EXT4_I(inode)->i_flags & EXT4_EA_INODE_FL)) {
+	if (!ext4_has_feature_ea_inode(inode->i_sb)) {
 		int ea_blocks = EXT4_I(inode)->i_file_acl ?
 				EXT4_CLUSTER_SIZE(inode->i_sb) >> 9 : 0;