vm_fault: try to only share-busy page for soft faults

If the fault handler found a vaild page that is definitely not going to be
renamed for COW, try to only sbusy the page. We do not need to validate
the page, and parallel faults on the same address are excluded by the
xbusy state of the page from the top object.

Reviewed by: alc, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D51474