LU-9859 libcfs: simplify capability dropping.

LU-9859 libcfs: simplify capability dropping.

Lustre has a 'squash credentials' concept similar to the "anon_uid"
for nfsd.  When accessing a file with squashed credentials, we
need to also drop capabilities.
Linux has cap_drop_fs_set() and cap_drop_nfsd_set().  Rather than
taking a completely different approach, this patch changes lustre
to use this same cap_drop_*_set() approach.

With this change we also drop CAP_MKNOD and CAP_MAC_OVERRIDE
which are probably appropriate, and don't drop
CAP_SYS_ADMIN or CAP_SYS_BOOT which should be irrelevant for
file permission checking

Calling both cap_drop_*_set() seems a bit clumsy, but gets
the job done.

Linux-commit: f497115d4cf8a430c5d9902ce35716ba5f9c21ef

Change-Id: I2f4f691bc4ad090f6abaa4e13eb473bf8d904b23
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://review.whamcloud.com/41957
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>