nfsdcld: Before clearing the capability bounding set, check if we have the cap

nfsdcld: Before clearing the capability bounding set, check if we have the cap

From: Harald Hoyer <harald@redhat.com>

PR_CAPBSET_DROP can return EINVAL, if an older kernel does support
some capabilities, which are defined by CAP_LAST_CAP, which results in
a failure of the service.

For example kernel 3.4 errors on CAP_EPOLLWAKEUP, which was newly
introduced in 3.5.

So, for future capabilities, we clear until we get an EINVAL for
PR_CAPBSET_READ.

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>