lib/generic-radix-tree.c: remove unneeded __rcu

lib/generic-radix-tree.c: remove unneeded __rcu

struct __genradix is defined as having its member 'root'
annotated as __rcu. But in the corresponding API RCU is not used.
Sparse reports this type mismatch as:
	lib/generic-radix-tree.c:56:35: warning: incorrect type in initializer (different address spaces)
	lib/generic-radix-tree.c:56:35:    expected struct genradix_root *r
	lib/generic-radix-tree.c:56:35:    got struct genradix_root [noderef] <asn:4> *__val
with 6 other ones.

So, correct root's type by removing this unneeded __rcu.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Link: http://lkml.kernel.org/r/20200621161745.55396-1-luc.vanoostenryck@gmail.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>