MFC: r238621

MFC: r238621

Revert the use of BUS_DMA_ALLOCNOW when creating the DMA tag for user
data introduced in r236061 (MFC'ed to stable/9 in r237186). 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]
Approved by:	re (kib)

git-svn-id: svn://svn.freebsd.org/base/stable/9@238689 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f