proc_sysctl.c: use BUG_ON instead of BUG

The use of if (!head) BUG(); can be replaced with the single line
BUG_ON(!head).

Signed-off-by: Prasad Joshi <prasadjoshi.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>