f2fs: Add F2FS_F_SB()

This is the folio equivalent of F2FS_P_SB().  Removes a call to
page_file_mapping() as we know folios seen by f2fs are never part of
the swap cache.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>