Linux 3.6 compat, kern_path_locked() added

Linux 3.6 compat, kern_path_locked() added

The kern_path_parent() function was removed from Linux 3.6 because
it was observed that all the callers just want the parent dentry.
The simpler kern_path_locked() function replaces kern_path_parent()
and does the lookup while holding the ->i_mutex lock.

This is good news for the vn implementation because it removes the
need for us to handle the locking.  However, it makes it harder to
implement a single readable vn_remove()/vn_rename() function which
is usually what we prefer.

Therefore, we implement a new version of vn_remove()/vn_rename()
for Linux 3.6 and newer kernels.  This allows us to leave the
existing working implementation untouched, and to add a simpler
version for newer kernels.

Long term I would very much like to see all of the vn code removed
since what this code enabled is generally frowned upon in the kernel.
But that can't happen util we either abondon the zpool.cache file
or implement alternate infrastructure to update is correctly in
user space.

Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
Signed-off-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #154