xfs: use xfs_defer_alloc a bit more

Noticed by inspection, simple factoring allows the same allocation
routine to be used for both transaction and recovery contexts.

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