xfs_quota: fix unsigned int id comparisons

xfs_quota: fix unsigned int id comparisons

Fix compiler warnings about unsigned int comparisons by replacing them
with an explicit check for the one possible invalid value (-1U).
id_from_string sets exitcode to nonzero when it sees this value, so the
call sites don't have to do that.

Coverity-id: 1463855, 1463856, 1463857
Fixes: 67a73d6139d0 ("xfs_quota: refactor code to generate id from name")
Fixes: 36dc471cc9bb ("xfs_quota: allow individual timer extension")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>