hyperv/storvsc: Fix busdma constraints

• The BUS_DMA_KEEP_PG_OFFSET flag is needed, since storvsc_xferbuf_prepare() assumes that only the first segment may have a non-zero offset, and that all following segments are page-sized and -aligned.
• storvsc_xferbuf_prepare() handles 64-bit bus addresses, so avoid unneeded bouncing on i386.

PR: 285681
Reported by: dim
Tested by: dim, whu
MFC after: 2 weeks