nfsd4: fix struct file leak

Make sure we properly reference count the struct files that a lock
depends on, and release them when the lock stateid is released.

This fixes a major leak of struct files when using locking over nfsv4.

Cc: stable@kernel.org
Reported-by: Rick Koshi <nfs-bug-report@more-right-rudder.com>
Tested-by: Ivo Přikryl <prikryl@eurosat.cz>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>