vm_object: Make is_object_active() global

Commit 867c27c23a5c modified the NFS client so that
it does IO_APPEND writes directly to the NFS server,
bypassing the buffer cache. However, this could result
in stale data in client pages when the file is mmap(2)'d.
As such, the NFS client needs to call is_object_active()
to check if the file is mmap(2)'d.

This patch renames is_object_active() to vm_object_is_active(),
moves it to sys/vm/vm_object.c and makes it global, so that
the NFS client can call it in a future commit.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33520