xfs: remove dead error handling code in xfs_dquot_disk_alloc()

Colin Ian King reports that commit 82ff27bc ("xfs: automatic dfops
buffer relogging") leaves around some dead error handling code in
xfs_dquot_disk_alloc(). This was discovered via Coverity scan.

Since the associated commit eliminates the act of joining a buffer
to a dfops, this intermediate error state is no longer possible and
the error handling code can be removed. Since the caller cancels the
transaction on error, which cancels the dfops, eliminate the
unnecessary xfs_defer_cancel() call and error handling labels.

Fixes: 82ff27bc ("xfs: automatic dfops buffer relogging")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>