LU-13783 libcfs: switch from ->mmap_sem to mmap_lock()

LU-13783 libcfs: switch from ->mmap_sem to mmap_lock()

In Linux 5.8, ->mmap_sem is gone and the preferred interface
for locking the mmap is to suite of mmap*lock() functions.

So provide those functions when not available, and use them
as needed in Lustre.

Lustre-change: https://review.whamcloud.com/40288
Lustre-commit: 5309e108582c692f3b60705818fddc4a3b3b1345

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I4ce3959f9e93eae10a7b7db03e2b0a1525723138
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/43643
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>