radix tree test suite: allow testing other fan-out values

The defines in regression2.c are already in radix-tree.h and duplicating
them in the test case makes experimenting with other values for the
fan-out harder than necessary.  Allow the user of the radix tree to
decide what the fan-out should be rather than fixing it to 8 for
non-kernel uses.

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Kirill Shutemov <kirill.shutemov@linux.intel.com>
Cc: Jan Kara <jack@suse.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>