Reset l2ad_hand and l2ad_first in l2arc_evict

Increasing l2arc_write_size or l2arc_write_boost can result in
l2arc_write_buffers() not having enough space to perform its writes and
panic zio_write_phys().

Instead of resetting l2ad_hand to l2ad_start at the end of
l2arc_write_buffers() and not taking into account a possible
user-mediated increase of l2arc_write_max, we do this in l2arc_evict(),
right after l2arc_write_size() has run. If there is not enough space to
evict (ie we will exceed l2ad_end) we evict to the end of the device,
reset l2ad_hand to l2ad_start, set l2ad_first to 0 and iterate
l2arc_evict(). We avoid infinite iteration of l2arc_evict() by making
sure in l2arc_write_size() that l2ad_start + size does not exceed
l2ad_end.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #10154

tests/zfs-tests/include/libtest.shlib

@@ -4053,3 +4053,16 @@ function ls_xattr # path
 		;;
 	esac
 }
+
+function get_arcstat # stat
+{
+	if is_linux; then
+		typeset stat=$1
+		typeset zfs_arcstats="/proc/spl/kstat/zfs/arcstats"
+		[[ -f "$zfs_arcstats" ]] || return 1
+		grep $stat $zfs_arcstats | awk '{print $3}'
+		return $?
+	else
+		return 1
+	fi
+}