blkcg: inline [__]blkg_lookup()

blkg_lookup() checks whether the target queue is bypassing and, if
not, calls __blkg_lookup() which first checks the lookup hint and then
performs radix tree walk.  The operations upto hint checking are
trivial and there are many users of this function.  This patch inlines
blkg_lookup() and the fast path part of __blkg_lookup().  The radix
tree lookup and hint update are now in blkg_lookup_slowpath().

This will help consolidating blkg handling by easing moving root blkcg
short-circuit to inlined lookup fast path.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Arianna Avanzini <avanzini.arianna@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>