zfs: Fix return value of fs_devread()

As evidenced by how other filesystems handle it, a return value of 0
from fs_devread() means failure; nonzero means success. The opposite
assumption was being made in zfs.c for the use of zfs_devread() so fix
the confusion by making zfs_devread() return 0 on success.

It probably doesn't make sense to change the handling of zfs_devread()
in zfs.c instead, because as it is it matches the semantics of the other
functions there.

Signed-off-by: Phaedrus Leeds <mwleeds@mailtundra.com>