fs: fat: fix link error when building with DEBUG=1

When compiling with DEBUG=1 an error
fs/fat/fat_write.c:831: undefined reference to `__aeabi_ldivmod'
occurred.

We should use do_div() instead of the modulus operator.

filesize and cur_pos cannot be negative. So let's use u64 to avoid
warnings.

Fixes: cb8af8af ("fs: fat: support write with non-zero offset")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>