fs: fat: avoid multiplication overflow

The product of two 32 bit integers is a 32 bit integer. Hence
clustcount * bytesperclust may overflow on > 4 GiB devices.

Change the type of clustcount.

Fixes: cb8af8af ("fs: fat: support write with non-zero offset")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>