fs: zfs: fix illegal use of fp

the upcoming sunxi (allwinner a10/a13) platform enables zfs
by default, and using linaro's hf -msoft-float makes the build
fail because this u64 division.

Signed-off-by: Alejandro Mery <amery@geeks.cl>
Acked-by: Stefan Roese <sr@denx.de>