swap_pager: speedup meta_transfer

Add a parameter to swp_pager_meta_build, for the benefit of
swp_pager_meta_transfer.

swp_pager_meta_transfer calls swp_pager_xfer_source, which may look up
the same trie entry twice - first, by calling sw_pager_meta_lookup,
and then as the first step in swp_pager_meta_build. A boolean
parameter to swp_pager_meta_build tells that function not to replace a
previously assigned swapblk with a new one, and setting it in this
call makes the first meta_lookup call unnecessary.

swp_pager_meta_transfer calls swp_pager_xfer_source, which may release
and reacquire the source object write lock, because the call to
swp_pager_meta_build may acquire and then release the destination
object write block. But it probably doesn't, so fiddling with the
source object write block was probably unnecessary. This boolean
parameter to swp_pager_meta_build tells it to return immediately if
memory allocation problems are about to require a lock
release/reacquisitiion, so that the caller can release/reacquire the
source object write lock only if truly necessary, around a second call
the swp_pager_meta_build with that boolean parameter not set. This
should make manipulation of the source object write lock rarer.

Reviewed by: alc, kib (previous version)
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D45781