vol: Free vol header on attach_volume_header error

vol: Free vol header on attach_volume_header error

In attach_volume_header, make sure we free the volume's header if
we're returning an error. We take care of the locks and i/o handles in
the immediately preceding block, but for an actual error we don't get
rid of the header. Do so.

Noticed by Tom Keiser.

Reviewed-on: http://gerrit.openafs.org/7325
Reviewed-by: Tom Keiser <tkeiser@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit ecfd9549fc29cdad8042e830c656caee1363c6cf)

Change-Id: I1d49c18f23cd576dfcde114238ea2b41e1356039
Reviewed-on: http://gerrit.openafs.org/7492
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>