ip_set.c:42:8: warning: type defaults to 'int' in declaration of
'DECLARE_MUTEX'
2.6.31-rt has cleaned up the naming mess in semaphore.h. Without
making use of #if hackery, we can use struct semaphore as declaration
plus an explicit sema_init() call.
As of Linux kernel commit 2351ec533ed0dd56052ab96988d2161d5ecc8ed9,
semaphore.h was moved from asm/ to linux/, which breaks building of
ipset. Add compat glue to ip_set.c to fix building on 2.6.27 an
onwards.
