ktls: Cache output buffers for software encryption

Maintain a cache of physically contiguous runs of pages for use as
output buffers when software encryption is configured and in-place
encryption is not possible. This makes allocation and free cheaper
since in the common case we avoid touching the vm_page structures for
the buffer, and fewer calls into UMA are needed. gallatin@ reports a
~10% absolute decrease in CPU usage with sendfile/KTLS on a Xeon after
this change.

It is possible that we will not be able to allocate these buffers if
physical memory is fragmented. To avoid frequently calling into the
physical memory allocator in this scenario, rate-limit allocation
attempts after a failure. In the failure case we fall back to the old
behaviour of allocating a page at a time.

N.B.: this scheme could be simplified, either by simply using malloc()
and looking up the PAs of the pages backing the buffer, or by falling
back to page by page allocation and creating a mapping in the cache
zone. This requires some way to save a mapping of an M_EXTPG page array
in the mbuf, though. m_data is not really appropriate. The second
approach may be possible by saving the mapping in the plinks union of
the first vm_page structure of the array, but this would force a vm_page
access when freeing an mbuf.

Reviewed by: gallatin, jhb
Tested by: gallatin
Sponsored by: Ampere Computing
Submitted by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D28556