fs/fat: merge readwrite get_fatent_value() with readonly get_fatent()

get_fatent_value(...) flushes changed FAT entries to disk when fetching
the next FAT blocks, in every other aspect it is identical to
get_fatent(...).

Provide a stub implementation for flush_dirty_fat_buffer if
CONFIG_FAT_WRITE is not set. Calling flush_dirty_fat_buffer during read
only operation is fine as it checks if any buffers needs flushing.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Reviewed-by: Benoît Thébaudeau <benoit.thebaudeau.dev@gmail.com>