nfsd: rework hashtable handling in nfsd_do_file_acquire

nfsd_file is RCU-freed, so we need to hold the rcu_read_lock long enough
to get a reference after finding it in the hash. Take the
rcu_read_lock() and call rhashtable_lookup directly.

Switch to using rhashtable_lookup_insert_key as well, and use the usual
retry mechanism if we hit an -EEXIST. Rename the "retry" bool to
open_retry, and eliminiate the insert_err goto target.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>