[SPARC64]: Fix _PAGE_EXEC_4U check in sun4u I-TLB miss handler.

It was using an immediate _PAGE_EXEC_4U value in an 'and'
instruction to perform the test.  This doesn't work because
the immediate field is signed 13-bit, this the mask being
tested against the PTE was 0x1000 sign-extended to 32-bits
instead of just plain 0x1000.

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