LU-14787 libcfs: Proved an abstraction for AS_EXITING

LU-14787 libcfs: Proved an abstraction for AS_EXITING

Linux kernel v3.14-7405-g91b0abe36a7b added AS_EXITING flag
AS_EXITING flag is set while address_space mapping is exiting.

Provide an abstraction mapping_clear_exiting() to clear
the AS_EXITING flag. This balances the kernel mapping_set_existing()
and is used for older kernels when enum mapping_flags does
not include AS_EXITING.

HPE-bug-id: LUS-9977
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: Ib3101b7e3eb8a7fcfd0012ac27367f1e65537f5d
Reviewed-on: https://review.whamcloud.com/44070
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>