fs: fat: update parent dirs metadata on dentry create/delete

fs: fat: update parent dirs metadata on dentry create/delete

POSIX filesystem functions that create or remove directory entries contain
text along the lines of "[function] shall mark for update the last data
modification and last file status change timestamps of the parent
directory of each file." [1][2][3] The common theme is these timestamp
updates occur when a directory entry is added or removed. The
create_link() and delete_dentry_link() functions have been changed to
update the modification timestamp on the directory where the direntry
change occurs. This differs slightly from Linux in the case of rename(),
where Linux will not update `new_path`'s parent directory's timestamp if
it is replacing an existing file. (via `vfat_add_entry` [4])

The timestamps are not updated if the build configuration does not support
RTCs. This is an effort to minimize introducing erratic timestamps where
they would go from [current date] -> 2000-01-01 (error timestamp in the
FAT driver). I would assume an unchanged timestamp would be more valuable
than a default timestamp in these cases.

[1] https://pubs.opengroup.org/onlinepubs/9799919799/functions/rename.html
[2] https://pubs.opengroup.org/onlinepubs/9799919799/functions/unlink.html
[3] https://pubs.opengroup.org/onlinepubs/9799919799/functions/open.html
[4] https://elixir.bootlin.com/linux/v6.12.6/source/fs/fat/namei_vfat.c#L682

Signed-off-by: Gabriel Dalimonte <gabriel.dalimonte@gmail.com>