checkpatch: add __rcu as a sparse modifier

Fix "need consistent spacing around '*'" error after a __rcu sparse
annotation which was caused by the missing __rcu entry in the
checkpatch.pl internal list of sparse keywords.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>