f2fs: introduce F2FS_I_SB, F2FS_M_SB, and F2FS_P_SB

This patch adds three inline functions to clean up dirty casting codes.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/recovery.c b/fs/f2fs/recovery.c
index 756c41c..5c095f7 100644
--- a/fs/f2fs/recovery.c
+++ b/fs/f2fs/recovery.c
@@ -75,7 +75,7 @@
 				err = -EEXIST;
 			goto out_unmap_put;
 		}
-		err = acquire_orphan_inode(F2FS_SB(inode->i_sb));
+		err = acquire_orphan_inode(F2FS_I_SB(inode));
 		if (err) {
 			iput(einode);
 			goto out_unmap_put;