smbfs: use dget_parent

Use dget_parent instead of opencoding it.  This simplifies the code, but
more importanly prepares for the more complicated locking for a parent
dget in the dcache scale patch series.

Note that the d_time assignment in smb_renew_times moves out of d_lock,
but it's a single atomic 32-bit value, and that's what other sites
setting it do already.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>