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: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>