xfs: use xfs_alloc_vextent_first_ag() where appropriate

Change obvious callers of single AG allocation to use
xfs_alloc_vextent_first_ag(). This gets rid of
XFS_ALLOCTYPE_FIRST_AG as the type used within
xfs_alloc_vextent_first_ag() during iteration is _THIS_AG. Hence we
can remove the setting of args->type from all the callers of
_first_ag() and remove the alloctype.

While doing this, pass the allocation target fsb as a parameter
rather than encoding it in args->fsbno. This starts the process
of making args->fsbno an output only variable rather than
input/output.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>