Move __set/clear_bit from ubifs.h to bitops.h

__set_bit and __clear_bit are defined in ubifs.h as well as in
asm/include/bitops.h for some architectures. This patch moves
the generic implementation to include/linux/bitops.h and uses
that unless it's defined by the architecture.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>