f2fs: decouple F2FS_MAP_ from buffer head flags

m_flags is never interchanged with the buffer_heads b_flags directly,
so use separate codepoints from that.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>