fs: xattr_handler table should be const

The entries in xattr handler table should be immutable (ie const)
like other operation tables.

Later patches convert common filesystems. Uncoverted filesystems
will still work, but will generate a compiler warning.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/xattr.h

@@ -37,7 +37,7 @@ struct inode;
 struct dentry;
 
 struct xattr_handler {
-	char *prefix;
+	const char *prefix;
 	int flags;	/* fs private flags passed back to the handlers */
 	size_t (*list)(struct dentry *dentry, char *list, size_t list_size,
 		       const char *name, size_t name_len, int handler_flags);