xfs: distinguish between corrupt inode and invalid inum in xfs_scrub_get_inode

In xfs_scrub_get_inode, we don't do a good enough job distinguishing
EINVAL returns from xfs_iget w/ IGET_UNTRUSTED -- this can happen if the
passed in inode number is invalid (past eofs, inobt says it isn't an
inode) or if the inum is actually valid but the inode buffer fails
verifier.  In the first case we still want to return ENOENT, but in the
second case we want to capture the corruption error.

Therefore, if xfs_iget returns EINVAL, try the raw imap lookup.  If that
succeeds, we conclude it's a corruption error, otherwise we just bounce
out to userspace.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>