lockd: push lock_flocks down

lockd should use lock_flocks() instead of lock_kernel()
to lock against posix locks accessing the i_flock list.

This is a prerequisite to turning lock_flocks into a
spinlock.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: J. Bruce Fields <bfields@redhat.com>