linuxkpi: Add mmap_lock.h

This contain mmap_read_lock, mmap_read_unlock and mmap_write_lock_killable
which are abstraction around down_read, up_read and down_write_killable.
Note that in Linux 5.8 mmap_sem was renamed to mmap_lock.
We might want to do the same at some point but some drivers still uses
the old mmap locking API.

Reviewed by: bz
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D34297

(cherry picked from commit 995c3b88d485b9b64ab94efe270d424b2526ade7)