Assorted fixes for the performance tests

- Bail out early if we're running the perf tests and forget to
  specify disks.
- Allow perf tests to run with any number of disks.
- Remove weekly vs. nightly settings
- Move variables with common values to perf.shlib
- Use zinject to clear the ARC over export/import
- Fix dbuf cache size calculation

When the meaning of `dbuf_cache_max_bytes` changed, the performance
test that covers the dbuf cache started to fail. The test would try to
write files for the test using the max possible size of the cache,
inevitably filling the pool and failing. This change uses
`dbuf_cache_shift` to correctly calculate the dbuf cache size.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #12408

scripts/zfs-tests.sh

@@ -567,18 +567,17 @@ fi
 
 . "$STF_SUITE/include/default.cfg"
 
-msg
-msg "--- Configuration ---"
-msg "Runfiles:        $RUNFILES"
-msg "STF_TOOLS:       $STF_TOOLS"
-msg "STF_SUITE:       $STF_SUITE"
-msg "STF_PATH:        $STF_PATH"
-
 #
 # No DISKS have been provided so a basic file or loopback based devices
 # must be created for the test suite to use.
 #
 if [ -z "${DISKS}" ]; then
+	#
+	# If this is a performance run, prevent accidental use of
+	# loopback devices.
+	#
+	[ "$TAGS" = "perf" ] && fail "Running perf tests without disks."
+
 	#
 	# Create sparse files for the test suite.  These may be used
 	# directory or have loopback devices layered on them.
@@ -619,8 +618,14 @@ if [ -z "${DISKS}" ]; then
 	fi
 fi
 
+#
+# It may be desirable to test with fewer disks than the default when running
+# the performance tests, but the functional tests require at least three.
+#
 NUM_DISKS=$(echo "${DISKS}" | awk '{print NF}')
-[ "$NUM_DISKS" -lt 3 ] && fail "Not enough disks ($NUM_DISKS/3 minimum)"
+if [ "$TAGS" != "perf" ]; then
+	[ "$NUM_DISKS" -lt 3 ] && fail "Not enough disks ($NUM_DISKS/3 minimum)"
+fi
 
 #
 # Disable SELinux until the ZFS Test Suite has been updated accordingly.
@@ -637,6 +642,12 @@ if [ -e /sys/module/zfs/parameters/zfs_dbgmsg_enable ]; then
 	sudo /bin/sh -c "echo 0 >/proc/spl/kstat/zfs/dbgmsg"
 fi
 
+msg
+msg "--- Configuration ---"
+msg "Runfiles:        $RUNFILES"
+msg "STF_TOOLS:       $STF_TOOLS"
+msg "STF_SUITE:       $STF_SUITE"
+msg "STF_PATH:        $STF_PATH"
 msg "FILEDIR:         $FILEDIR"
 msg "FILES:           $FILES"
 msg "LOOPBACKS:       $LOOPBACKS"