Fix inconsistent locking of the swap pager named objects list.

Right now, all modifications of the list are locked by sw_alloc_mtx.
But initial lookup of the object by the handle in swap_pager_alloc()
is not protected by sw_alloc_mtx, which means that
vm_pager_object_lookup() could follow freed pointer.

Create a new named swap object with the OBJT_SWAP type, instead
of OBJT_DEFAULT. With this change, swp_pager_meta_build() never need
to upgrade named OBJT_DEFAULT to OBJT_SWAP (in the other place, we do
not forbid for client code to create named OBJT_DEFAULT objects at
all).

That change allows to remove sw_alloc_mtx and make the list locked by
sw_alloc_sx lock. Update swap_pager_copy() to new locking mode.

Create helper swap_pager_alloc_init() to consolidate named and
anonymous swap objects creation, while a caller ensures that the
neccesary locks are held around the helper.

Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Approved by: re (hrs)