Add test for stale seq in merge delta combining.

merge_read_item() fails to update found->seq when combining delta items
from multiple finalized log trees. Add a test case to replicate the
conditions of this issue.

Each of 5 mounts sets totl value 1 on 2500 shared keys, giving an
expected total of 5 per key.  Any total > 5 proves double-counting
from a stale seq.

The log_merge_force_partial trigger forces many partial merges per
cycle, creating the conditions where stale-seq items get spliced into
fs_root while finalized logs still exist.  Parallel readers on all
mounts race against this window to detect double-counted values.

Signed-off-by: Auke Kok <auke.kok@versity.com>

tests/golden/totl-merge-read

@@ -0,0 +1,3 @@
+== setup
+expected 4681
+== cleanup

tests/sequence

@@ -27,6 +27,7 @@ simple-xattr-unit.sh
 retention-basic.sh
 totl-xattr-tag.sh
 quota.sh
+totl-merge-read.sh
 lock-refleak.sh
 lock-shrink-consistency.sh
 lock-shrink-read-race.sh

tests/tests/totl-merge-read.sh

@@ -0,0 +1,50 @@
+#
+# Test that merge_read_item() correctly updates the sequence number when
+# combining delta items from multiple finalized log trees.  Each mount
+# sets a totl value in its own 3-bit lane (powers of 8) so that any
+# double-counting overflows the lane and is caught by: or(v, exp) != exp.
+#
+
+t_require_commands setfattr scoutfs
+t_require_mounts 5
+
+echo "== setup"
+for nr in $(t_fs_nrs); do
+	d=$(eval echo \$T_D$nr)
+	for i in $(seq 1 2500); do : > "$d/f$nr$i"; done
+done
+sync
+t_force_log_merge
+
+vals=(1 8 64 512 4096)
+expected=4681
+n=0
+for nr in $(t_fs_nrs); do
+	d=$(eval echo \$T_D$nr)
+	v=${vals[$((n++))]}
+	for i in $(seq 1 2500); do
+		setfattr -n "scoutfs.totl.t.$i.0.0" -v $v "$d/f$nr$i"
+	done
+done
+
+t_trigger_arm_silent log_merge_force_partial $(t_server_nr)
+
+bad="$T_TMPDIR/bad"
+for nr in $(t_fs_nrs); do
+	( while true; do
+		echo 1 > "$(t_debugfs_path $nr)/drop_weak_item_cache"
+		scoutfs read-xattr-totals -p "$(eval echo \$T_M$nr)" | \
+			awk -F'[ =,]+' -v e=$expected 'or($2+0,e) != e'
+	done ) >> "$bad" &
+done
+
+echo "expected $expected"
+t_force_log_merge
+t_silent_kill $(jobs -p)
+test -s "$bad" && echo "double-counted:" && cat "$bad"
+
+echo "== cleanup"
+for nr in $(t_fs_nrs); do
+	find "$(eval echo \$T_D$nr)" -name "f$nr*" -delete
+done
+t_pass