xfs: constify xfs_name_dotdot

The symbol xfs_name_dotdot is a global variable that the xfs codebase
uses here and there to look up directory dotdot entries.  Currently it's
a non-const variable, which means that it's a mutable global variable.
So far nobody's abused this to cause problems, but let's use the
compiler to enforce that.
Signed-off-by: NDarrick J. Wong <djwong@kernel.org>
Reviewed-by: NDave Chinner <dchinner@redhat.com>

fs/xfs/libxfs/xfs_dir2.c

@@ -19,7 +19,11 @@
 #include "xfs_error.h"
 #include "xfs_trace.h"
 
-struct xfs_name xfs_name_dotdot = { (unsigned char *)"..", 2, XFS_DIR3_FT_DIR };
+const struct xfs_name xfs_name_dotdot = {
+	.name	= (const unsigned char *)"..",
+	.len	= 2,
+	.type	= XFS_DIR3_FT_DIR,
+};
 
 /*
  * Convert inode mode to directory entry filetype

fs/xfs/libxfs/xfs_dir2.h

@@ -21,7 +21,7 @@ struct xfs_dir2_data_unused;
 struct xfs_dir3_icfree_hdr;
 struct xfs_dir3_icleaf_hdr;
 
-extern struct xfs_name	xfs_name_dotdot;
+extern const struct xfs_name	xfs_name_dotdot;
 
 /*
  * Convert inode mode to directory entry filetype