Rename struct rb_node into struct fio_rb_node

Rename struct rb_node into struct fio_rb_node

This patch avoids that building fio fails as follows on NetBSD systems:

    CC crc/test.o
In file included from crc/../iolog.h:4,
                 from crc/../stat.h:4,
                 from crc/../thread_options.h:7,
                 from crc/../fio.h:18,
                 from crc/test.c:5:
crc/../lib/rbtree.h:101: error: redefinition of 'struct rb_node'
Makefile:336: recipe for target 'crc/test.o' failed
gmake: *** [crc/test.o] Error 1

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>