Revert the use of BUS_DMA_ALLOCNOW when creating the DMA tag for user

Revert the use of BUS_DMA_ALLOCNOW when creating the DMA tag for user
data introduced in r236061. Using that flag doesn't make that much
sense on this case as the DMA maps using it are also created during
sym_pci_attach(). Moreover, due to the maxsegsz parameter used, doing
so may exhaust the bounce pages pool on architectures requiring
bounce pages. [1]
While at it, use a slightly more appropriate maxsegsz parameter.

PR:		169526
Submitted by:	Mike Watters [1]
MFC after:	3 days