[SPARC64]: Fix pfn_pte() build failure.

The "%uhi" needs to be "%%uhi" because we want a real
"%" character in the assembler here, instead of an
assembler variable expansion.

Aparently older GCCs were more liberal and interpreted
this %-letter as a literal "%" for whatever reason.

Based upon a build failure report from Meelis Roos.

Signed-off-by: David S. Miller <davem@davemloft.net>