Implement local sfbuf_map and sfbuf_unmap for MIPS32.

The pre-rework behaviour was not to keep the cached mappings around after
the sfbuf was used but instead to recycle said mappings.

PR:		kern/193400


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271213 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f