ext4: bubble errors from ext4_find_inline_data_nolock() up to ext4_iget()

If ext4_find_inline_data_nolock() returns an error it needs to get
reflected up to ext4_iget().  In order to fix this,
ext4_iget_extra_inode() needs to return an error (and not return
void).

This is related to "ext4: do not allow external inodes for inline
data" (which fixes CVE-2018-11412) in that in the errors=continue
case, it would be useful to for userspace to receive an error
indicating that file system is corrupted.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@kernel.org