Since size_t has the same size as a long on all architectures, it's enough
for overflow checks to check against ULONG_MAX.

This change could allow a compiler better optimization (especially in the
n=1 case).

The practical effect seems to be positive, but quite small:

    text           data     bss      dec            hex filename
21762380        5859870 1848928 29471178        1c1b1ca vmlinux-old
21762211        5859870 1848928 29471009        1c1b121 vmlinux-patched
Signed-off-by: NAdrian Bunk <bunk@stusta.de>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>