vfs: stop handling VI_OWEINACT in vget

vget is almost always called with LK_SHARED, meaning the flag (if present) is
almost guaranteed to get cleared. Stop handling it in the first place and
instead let the thread which wanted to do inactive handle the bumepd usecount.

Reviewed by: jeff
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D23184