We had some bugs with setting/using first_this_inode in the inode
walker in the dirents/xattr code.

This patch changes to not clear first_this_inode until after
initializing the new hash info.

Also, we fix an error message to not print on transaction restart, and
add a comment to related fsck error code.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>