sparc32: fix a user-triggerable oops in clear_user()

Back in 2.1.29 the clear_user() guts (__bzero()) had been merged
with memset().  Unfortunately, while all exception handlers had been
copied, one of the exception table entries got lost.  As the result,
clear_user() starting at 128*n bytes before the end of page and
spanning between 8 and 127 bytes into the next page would oops when
the second page is unmapped.  It's trivial to reproduce - all
it takes is

main()
{
	int fd = open("/dev/zero", O_RDONLY);
	char *p = mmap(NULL, 16384, PROT_READ|PROT_WRITE,
			MAP_PRIVATE|MAP_ANON, -1, 0);
	munmap(p + 8192, 8192);
	read(fd, p + 8192 - 128, 192);
}

which had been oopsing since March 1997.  Says something about
the quality of test coverage... ;-/  And while today sparc32 port
is nearly dead, back in '97 it had been very much alive; in fact,
sparc64 had only been in mainline for 3 months by that point...

Cc: stable@kernel.org
Fixes: v2.1.29
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>