ext4: clean up return values in handle_mount_opt()

Clean up return values in handle_mount_opt() and rename the function to
ext4_parse_param()

Now we can use it in fs_context_operations as .parse_param.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Link: https://lore.kernel.org/r/20211027141857.33657-11-lczerner@redhat.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>