ext4: move option validation to a separate function

Move option validation out of parse_options() into a separate function
ext4_validate_options().

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Link: https://lore.kernel.org/r/20211027141857.33657-4-lczerner@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>