fat: write: fix broken write at non-zero file offset

fat: write: fix broken write at non-zero file offset

Handling of the start file offset was broken in the current code. Although
the code skipped the needed clusters, it then tried to continue write with
current cluster set to EOF, what caused assertion. It also lacked adjusting
filesize in case of writing at the end of file and adjusting in-cluster
offset for partial overwrite.

This patch fixes all those issues.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I24cf908407ef951920291e560db28e603799ae77