xfs_scrub: fix xfrog_scrub_metadata error reporting

xfs_scrub: fix xfrog_scrub_metadata error reporting

Commit de5d20ec converted xfrog_scrub_metadata to return negative error
codes directly, but forgot to fix up the str_errno calls to use
str_liberror.  This doesn't result in incorrect error reporting
currently, but (a) the calls in the switch statement are inconsistent,
and (b) this will matter in future patches where we can call library
functions in between xfrog_scrub_metadata and str_liberror.

Fixes: de5d20ec ("libfrog: convert scrub.c functions to negative error codes")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>