MINOR: threads: augment rwlock debugging stats to report seek lock stats

We currently use only read and write lock operations with rwlocks, but
ours also support upgradable seek locks for which we do not report any
stats. Let's add them now when DEBUG_THREAD is enabled.
diff --git a/include/haproxy/thread.h b/include/haproxy/thread.h
index a9a4b62..a3451bd 100644
--- a/include/haproxy/thread.h
+++ b/include/haproxy/thread.h
@@ -422,16 +422,26 @@
 			"\t # write unlock: %lu (%ld)\n"
 			"\t # wait time for write     : %.3f msec\n"
 			"\t # wait time for write/lock: %.3f nsec\n"
+			"\t # seek lock   : %lu\n"
+			"\t # seek unlock : %lu (%ld)\n"
+			"\t # wait time for seek      : %.3f msec\n"
+			"\t # wait time for seek/lock : %.3f nsec\n"
 			"\t # read lock   : %lu\n"
 			"\t # read unlock : %lu (%ld)\n"
 			"\t # wait time for read      : %.3f msec\n"
 			"\t # wait time for read/lock : %.3f nsec\n",
+
 			lock_label(lbl),
 			lock_stats[lbl].num_write_locked,
 			lock_stats[lbl].num_write_unlocked,
 			lock_stats[lbl].num_write_unlocked - lock_stats[lbl].num_write_locked,
 			(double)lock_stats[lbl].nsec_wait_for_write / 1000000.0,
 			lock_stats[lbl].num_write_locked ? ((double)lock_stats[lbl].nsec_wait_for_write / (double)lock_stats[lbl].num_write_locked) : 0,
+			lock_stats[lbl].num_seek_locked,
+			lock_stats[lbl].num_seek_unlocked,
+			lock_stats[lbl].num_seek_unlocked - lock_stats[lbl].num_seek_locked,
+			(double)lock_stats[lbl].nsec_wait_for_seek / 1000000.0,
+			lock_stats[lbl].num_seek_locked ? ((double)lock_stats[lbl].nsec_wait_for_seek / (double)lock_stats[lbl].num_seek_locked) : 0,
 			lock_stats[lbl].num_read_locked,
 			lock_stats[lbl].num_read_unlocked,
 			lock_stats[lbl].num_read_unlocked - lock_stats[lbl].num_read_locked,