sparc64: Fix sun4u execute bit check in TSB I-TLB load.

Thanks to testcase and report from Brad Spengler:

--------------------
#include <stdio.h>

typedef int (* _wee)(void);

int main(void)
{
        char buf[8] = { '\x81', '\xc7', '\xe0', '\x08', '\x81', '\xe8',
                        '\x00', '\x00' };
        _wee wee;
        printf("%p\n", &buf);
        wee = (_wee)&buf;
        wee();

        return 0;
}
--------------------

TSB I-tlb load code tries to use andcc to check the _PAGE_EXEC_4U bit,
but that's bit 12 so it gets sign extended all the way up to bit 63
and the test nearly always passes as a result.

Use sethi to fix the bug.

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