Elide the object lock in the common case in vfs_vmio_unwire().

The object lock was only needed when attempting to free B_DIRECT
buffer pages, and for testing for invalid pages (and freeing them
if so). Handle the latter by instead moving invalid pages near the head
of the inactive queue, where they will be reclaimed quickly.

Reviewed by: alc, kib, jeff
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D14778