nlmsvc_lock calls nlmsvc_create_block with file->f_sema
held.
nlmsvc_create_block calls nlmclnt_lookup_host which might
call nlm_gc_hosts which might, eventually, try to claim
file->f_sema for the same file -> deadlock.

nlmsvc_create_block does not need any protection under
any lock as lockd is single-threaded and _create_block
only plays with internal data structures.

So we release the f_sema before calling in, and make sure
it gets claimed again afterwards.