LU-13783 libcfs: support __vmalloc with only 2 args.

LU-13783 libcfs: support __vmalloc with only 2 args.

Since v5.8-rc1~201^2~19 Commit 88dca4ca5a93 ("mm: remove the pgprot
argument to __vmalloc") __vmalloc only takes 2 arguments.

So introduce __ll_vmalloc which takes 2 args, and calls
__vmalloc with correct number of args.

Lustre-change: https://review.whamcloud.com/40328
Lustre-commit: 2a32eaa35dd7b96bb29f6a17991f48fe07fa833e

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I2c89512a12e28b27544a891620e448a9b752b089
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: Chris Horn <chris.horn@hpe.com>
Reviewed-on: https://review.whamcloud.com/43645
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>