fstests: Allow user to keep $seqres.dmesg for all tests

Currently fstests will remove $seqres.dmesg if nothing wrong
happened.  It saves some space, but sometimes it may not provide
good enough history for developers to check.

For example, some unexpected dmesg from fs, but not serious enough
to be caught by current filter.

So instead of deleting the ordinary $seqres.dmesg, provide a new
config: KEEP_DMESG, to allow user to keep the dmesg by setting it to
"yes".

The default value for it is "no", which keeps the existing behavior
by deleting ordinary dmesg.

[Eryu: change it to a "yes"/"no" switch.]

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

README

@@ -101,6 +101,7 @@ Preparing system for tests:
                using nvme drives.
              - set USE_KMEMLEAK=yes to scan for memory leaks in the kernel
                after every test, if the kernel supports kmemleak.
+             - set KEEP_DMESG=yes to keep dmesg log after test
 
         - or add a case to the switch in common/config assigning
           these variables based on the hostname of your test

common/config

@@ -22,6 +22,9 @@
 # RMT_IRIXTAPE_DEV- the IRIX remote tape device for the xfsdump tests
 # RMT_TAPE_USER -   remote user for tape device
 # SELINUX_MOUNT_OPTIONS - Options to use when SELinux is enabled.
+# KEEP_DMESG -      whether to keep all dmesg for each test case.
+#                   yes: keep all dmesg
+#                   no: only keep dmesg with error/warning (default)
 #
 # - These can be added to $HOST_CONFIG_DIR (witch default to ./config)
 #   below or a separate local configuration file can be used (using

common/rc

@@ -3634,7 +3634,9 @@ _check_dmesg()
 		_dump_err "_check_dmesg: something found in dmesg (see $seqres.dmesg)"
 		return 1
 	else
+		if [ "$KEEP_DMESG" != "yes" ]; then
 			rm -f $seqres.dmesg
+		fi
 		return 0
 	fi
 }