xfs: try to idiot-proof the allocators

In porting his development branch to 6.3-rc1, yours truly has
repeatedly screwed up the args->pag being fed to the xfs_alloc_vextent*
functions.  Add some debugging assertions to test the preconditions
required of the callers.

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