nfsd: allow filecache open to skip fh_verify check

nfsd: allow filecache open to skip fh_verify check

Currently, we call fh_verify twice on the filehandle. Once when we call
into nfsd_file_acquire, and then again from nfsd_open. The second one is
completely superfluous though, and fh_verify can do some things that
require a fair bit of work (checking permissions, for instance).

Create a new nfsd_open_verified function that will do an nfsd_open on a
filehandle that has already been verified. Call that from the filecache
code.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>