Export __locks_copy_lock() so modular lockd builds

Commit 1a747ee0 ("locks: don't call ->copy_lock methods on return of
conflicting locks") changed fs/lockd/svclock.c to call
__locks_copy_lock() instead of locks_copy_lock(), but lockd can be built
as a module and __locks_copy_lock() is not exported, which causes a
build error

    ERROR: "__locks_copy_lock" [fs/lockd/lockd.ko] undefined!

with CONFIG_LOCKD=m.

Fix this by exporting __locks_copy_lock().

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>