net: ipmr: remove some pimsm ifdefs and simplify

Add the helper pimsm_enabled() which replaces the old CONFIG_IP_PIMSM
define and is used to check if any version of PIM-SM has been enabled.
Use a single if defined(CONFIG_IP_PIMSM_V1) || defined(CONFIG_IP_PIMSM_V2)
for the pim-sm shared code. This is okay w.r.t IGMPMSG_WHOLEPKT because
only a VIFF_REGISTER device can send such packet, and it can't be
created if pimsm_enabled() is false.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>