dcache: export shrink_dentry_list() and add new helper d_dispose_if_unused()

dcache: export shrink_dentry_list() and add new helper d_dispose_if_unused()

Add and export a new helper d_dispose_if_unused() which is simply a wrapper
around to_shrink_list(), to add an entry to a dispose list if it's not used
anymore.

Also export shrink_dentry_list() to kill all dentries in a dispose list.

Suggested-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Luis Henriques <luis@igalia.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>