LU-9859 libcfs: simplify linux-prim.c

LU-9859 libcfs: simplify linux-prim.c

cfs_block_sigs() is never used.
cfs_clear_sigpending() is only used in lustre_lib.h so move it
to that header. Based on

Linux-commit: 99c1ffc99a570c68cef906d9763edb47b316ea1a

Change-Id: Ia0d5ecb736c4107c5a7b666bda85714d6819fbca
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/35410
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>