HGFS: smp_mb__before_clear_bit missing in 3.18

Fixes compile error on 3.18

vmhgfs-only/page.c:1681:4: error: implicit declaration of function ‘smp_mb__before_clear_bit’ 

Disappeared in 3.18, but already in 3.17 an alternative name is given:

http://lxr.free-electrons.com/source/include/linux/bitops.h?v=3.17#L40