Add sglist functions for working with arrays of vmpages.

[jhb]

Summary

Add sglist functions for working with arrays of vmpages.

sglist_count_vmpages() determines the number of segments required for
a buffer described by an array of VM pages. sglist_append_vmpages()
adds the segments described by such a buffer to an sglist. The latter
function is largely pulled from sglist_append_bio(), and
sglist_append_bio() now uses sglist_append_vmpages().

Test Plan

• booted a system to multiuser ok. Have also used the new functions in some pending aio_write zero-copy stuff for Chelsio TOE.

[jhb]

Committed in rS300337