Refactor generic inode time updating

ZFS doesn't provide a custom update_time method meaning it delegates
this job to the generic VFS layer. The only time when it needs to
set the various *time values is when the inode is being marshalled
to/from the disk. Do this by moving the relevant code from
zfs_inode_update_impl to zfs_node_alloc and zfs_rezget. As a result
from this change it is no longer necessary to have multiple versions
of the zfs_inode_update function - so just nuke them and leave only
one.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Nikolay Borisov <n.borisov.lkml@gmail.com>
Issue #227
Closes #4916