xfs: separate dquot repair into a separate function

Move the dquot repair code into a separate function and remove
XFS_QMOPT_DQREPAIR in favor of calling the helper directly.  Remove
other dead code because quotacheck is the only caller of DQREPAIR.

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