[PATCH] kbuild: fix buffer overflow in modpost

Jiri Benc <jbenc@suse.cz> reported that modpost would stop with SIGABRT if
used with long filepaths.
The error looked like:
>   Building modules, stage 2.
>   MODPOST
> *** glibc detected *** scripts/mod/modpost: realloc(): invalid next size:
+0x0809f588 ***
> [...]

Fix this by allocating at least the required memory + SZ bytes each time.
Before we sometimes ended up allocating too little memory resuting in the
glibc detected bug above.  Based on patch originally submitted by: Jiri
Benc <jbenc@suse.cz>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>