libceph: Avoid holding the zero page on ceph_msgr_slab_init errors

ceph_msgr_slab_init may fail due to a temporary ENOMEM.

Delay a bit the initialization of zero_page in ceph_msgr_init and
reorder its cleanup in _ceph_msgr_exit so it's done in reverse
order of setup.

BUG_ON() will not suffer to be postponed in case it is triggered.

Signed-off-by: Benoît Canet <benoit.canet@nodalink.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>