f2fs: introduce IS_F2FS_IPU_* macro

IS_F2FS_IPU_* macro can be used to identify whether
f2fs ipu related policies are enabled.

BTW, convert to use BIT() instead of open code.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>