drivers: lustre: ptlrpc: check result of register_shrinker()

sptlrpc_enc_pool_init() does not check result of register_shrinker()
which was tagged __must_check recently, reported by sparse.

Signed-off-by: Aliaksei Karaliou <akaraliou.dev@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>