ext4: Use kstrtoul() instead of parse_strtoul()

In parse_strtoul() we're still using deprecated simple_strtoul().  Remove
parse_strtoul() altogether and replace it with kstrtoul()

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>