Fix remount races with symlink handling in affs

A couple of fields in affs_sb_info is used in follow_link() and
symlink() for handling AFFS "absolute" symlinks.  Need locking
against affs_remount() updates.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/affs/affs.h b/fs/affs/affs.h
index e511dc6..0e40caa 100644
--- a/fs/affs/affs.h
+++ b/fs/affs/affs.h
@@ -106,8 +106,8 @@
 	u32 s_last_bmap;
 	struct buffer_head *s_bmap_bh;
 	char *s_prefix;			/* Prefix for volumes and assigns. */
-	int s_prefix_len;		/* Length of prefix. */
 	char s_volume[32];		/* Volume prefix for absolute symlinks. */
+	spinlock_t symlink_lock;	/* protects the previous two */
 };
 
 #define SF_INTL		0x0001		/* International filesystem. */