btrfs: tracepoints: end assignment with semicolon at btrfs_qgroup_extent event class

While running checkpatch.pl against a patch that modifies the
btrfs_qgroup_extent event class, it complained about using a comma instead
of a semicolon:

  $ ./scripts/checkpatch.pl qgroups/0003-btrfs-qgroups-remove-bytenr-field-from-struct-btrfs_.patch
  WARNING: Possible comma where semicolon could be used
  #215: FILE: include/trace/events/btrfs.h:1720:
  +		__entry->bytenr		= bytenr,
		__entry->num_bytes	= rec->num_bytes;

  total: 0 errors, 1 warnings, 184 lines checked

So replace the comma with a semicolon to silence checkpatch and possibly
other tools. It also makes the code consistent with the rest.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>