ext4: Change handle_mount_opt() to use fs_parameter

Use the new mount option specifications to parse the options in
handle_mount_opt(). However we're still using the old API to get the
options string.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Link: https://lore.kernel.org/r/20211027141857.33657-5-lczerner@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>