Fix NFS credential

The commit f74b821 caused a regression where creating file through NFS will
always create a file owned by root. This is because the patch enables the KSID
code in zfs_acl_ids_create, which it would use euid and egid of the current
process. However, on Linux, we should use fsuid and fsgid for file operations,
which is the original behaviour. So we revert this part of code.

The patch also enables secpolicy_vnode_*, since they are also used in file
operations, we change them to use fsuid and fsgid.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4772
Closes #4758