kTLS: Fix a bug where we would not encrypt anon data inplace.

Software Kernel TLS needs to allocate a new destination crypto
buffer when encrypting data from the page cache, so as to avoid
overwriting shared clear-text file data with encrypted data
specific to a single socket. When the data is anonymous, eg, not
tied to a file, then we can encrypt in place and avoid allocating
a new page. This fixes a bug where the existing code always
assumes the data is private, and never encrypts in place. This
results in unneeded page allocations and potentially more memory
bandwidth consumption when doing socket writes.

When the code was written at Netflix, ktls_encrypt() looked at
private sendfile flags to determine if the pages being encrypted
where part of the page cache (coming from sendfile) or
anonymous (coming from sosend). This was broken internally at
Netflix when the sendfile flags were made private, and the
M_WRITABLE() check was added. Unfortunately, M_WRITABLE() will
always be false for M_NOMAP mbufs, since one cannot just mtod()
them.

This change introduces a new flags field to the mbuf_ext_pgs
struct by stealing a byte from the tls hdr. Note that the current
header is still 2 bytes larger than the largest header we
support: AES-CBC with explicit IV. We set MBUF_PEXT_FLAG_ANON
when creating an unmapped mbuf in m_uiotombuf_nomap() (which is
the path that socket writes take), and we check for that flag in
ktls_encrypt() when looking for anon pages.

Reviewed by: jhb
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D21796