Fix inconsistent locking of the swap pager named objects list.

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)