smbd: Fix coredump on failing chdir during logoff

smbd: Fix coredump on failing chdir during logoff

server_exit does an internal tree disconnect which requires a chdir to
the share directory. In case the file system encountered a problem and
the chdir call returns an error, this triggers a SERVER_EXIT_ABNORMAL
which in turn results in a panic and a coredump. As the log already
indicates the problem (chdir returned an error), avoid the
SERVER_EXIT_ABNORMAL in this case and not trigger a coredump.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13189

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Sat Dec 16 01:56:06 CET 2017 on sn-devel-144

(cherry picked from commit 7fa91fc4791d076c609eaf119753e38dd3c50a1c)

Autobuild-User(v4-6-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-6-test): Tue Jan  2 14:01:29 CET 2018 on sn-devel-144