staging: lustre: opencode cfs_cap_{raise, lower, raised}

Each of these functions is used precisely once, so having
a separate exported function seems like overkill.

cfs_cap_raised() is trivial - one line.
cfs_cap_raise() and cfs_cap_lower() are used as a pair
which is more effectively implemented with
override_cred() / revert_creds().

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>