include/linux/log2.h: add missing () around n in roundup_pow_of_two()

Otherwise gcc generates warnings if the expression is complicated.

Fixes: 312a0c17

 ("[PATCH] LOG2: Alter roundup_pow_of_two() so that it can use a ilog2() on a constant")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: https://lkml.kernel.org/r/0-v1-8a2697e3c003+41165-log_brackets_jgg@nvidia.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>