gfs2: add missing log_blocks trace points in gfs2_write_revokes

Function gfs2_write_revokes was incrementing and decrementing the number
of log blocks free, but there was never a log_blocks trace point for it.
Thus, the free blocks from a log_blocks trace would jump around
mysteriously.

This patch adds the missing trace points so the trace makes more sense.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>