watch_queue: Fix to release page in ->release()

watch_queue: Fix to release page in ->release()

BugLink: https://bugs.launchpad.net/bugs/1966056
commit c1853fbadcba1497f4907971e7107888e0714c81 upstream.

When a pipe ring descriptor points to a notification message, the
refcount on the backing page is incremented by the generic get function,
but the release function, which marks the bitmap, doesn't drop the page
ref.

Fix this by calling generic_pipe_buf_release() at the end of
watch_queue_pipe_buf_release().

Fixes: c73be61cede5 ("pipe: Add general notification queue support")
Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>