Fix statfs(2) for 32-bit user space

Fix statfs(2) for 32-bit user space

When handling a 32-bit statfs() system call the returned fields,
although 64-bit in the kernel, must be limited to 32-bits or an
EOVERFLOW error will be returned.

This is less of an issue for block counts since the default
reported block size in 128KiB. But since it is possible to
set a smaller block size, these values will be scaled as
needed to fit in a 32-bit unsigned long.

Unlike most other filesystems the total possible file counts
are more likely to overflow because they are calculated based
on the available free space in the pool. In order to prevent
this the reported value must be capped at 2^32-1. This is
only for statfs(2) reporting, there are no changes to the
internal ZFS limits.

Reviewed-by: Andreas Dilger <andreas.dilger@whamcloud.com>
Reviewed-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #7927
Closes #7122
Closes #7937