x86, UV: Use allocated buffer in tlb_uv.c:tunables_read()

The original code didn't check that the value returned from
snprintf() was less than the size of the buffer.  Although it
didn't cause a runtime bug in this case, it makes the static
checkers complain.

Andrew Morton suggested a dynamically sized buffer would be
cleaner.

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Cliff Wickman <cpw@sgi.com>
Cc: Jack Steiner <steiner@sgi.com>
Cc: Robin Holt <holt@sgi.com>
LKML-Reference: <20100929083118.GA6376@bicker>
Signed-off-by: Ingo Molnar <mingo@elte.hu>