net: ipmr: move pimsm_enabled to pim.h and rename

Move the inline pimsm_enabled() to pim.h and rename it to
ipmr_pimsm_enabled to show it's for the ipv4 ipmr code since pim.h is
used by IPv6 too.

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