vfs: Add sb_rdonly(sb) to query the MS_RDONLY flag on s_flags

Add an sb_rdonly() function to query the MS_RDONLY flag on sb->s_flags
preparatory to providing an SB_RDONLY flag.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

include/linux/fs.h

@@ -1844,7 +1844,8 @@ struct super_operations {
  */
 #define __IS_FLG(inode, flg)	((inode)->i_sb->s_flags & (flg))
 
-#define IS_RDONLY(inode)	((inode)->i_sb->s_flags & MS_RDONLY)
+static inline bool sb_rdonly(const struct super_block *sb) { return sb->s_flags & MS_RDONLY; }
+#define IS_RDONLY(inode)	sb_rdonly((inode)->i_sb)
 #define IS_SYNC(inode)		(__IS_FLG(inode, MS_SYNCHRONOUS) || \
 					((inode)->i_flags & S_SYNC))
 #define IS_DIRSYNC(inode)	(__IS_FLG(inode, MS_SYNCHRONOUS|MS_DIRSYNC) || \