btrfs: tracepoints: fix qgroup reservation type printing

Since qgroup's reservation types are define in a macro they must be
exported to user space in order for user space tools to convert raw
binary data to symbolic names. Currently trace-cmd report produces
the following output:

kworker/u8:2-459   [003]  1208.543587: qgroup_update_reserve:
2b742cae-e0e5-4def-9ef7-28a9b34a951e: qgid=5 type=0x2 cur_reserved=54870016 diff=-32768

With this fix the output is:

kworker/u8:2-459   [003]  1208.543587: qgroup_update_reserve:
2b742cae-e0e5-4def-9ef7-28a9b34a951e: qgid=5 type=BTRFS_QGROUP_RSV_META_PREALLOC cur_reserved=54870016 diff=-32768

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>