fstyp: Remove __packed from struct exfat_de_label.

This fixes a -Waddress-of-packed-member warning about a possibly
unaligned pointer from GCC 9 when calling convert_label().

__packed has to be removed from struct exfat_dirent as well to fix an
alignment warning when casting from a struct exfat_dirent pointer to a
struct exfat_de_label pointer.

Reviewed by: cem
Differential Revision: https://reviews.freebsd.org/D32144