xfs: constify dotdot global variable

I was auditing the code base when I noticed that the xfs_name_dotdot
variable is both global and mutable.  In theory, someone could change
the contents of that variable (either through misuse or by exploiting a
kernel bug) which would then break the directory code, so let's shut
down that attack surface by making it const.

v2: make the lookup function name argument const, thereby avoiding
    clunkiness in the second patch
-----BEGIN PGP SIGNATURE-----
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=kI4o
-----END PGP SIGNATURE-----