Fixed race on put_files_struct on exec with proc.
Restoring files on current on error path may lead
to proc having a pointer to already kfree-d files_struct.

->files changing at exit.c and khtread.c are safe as
exit_files() makes all things under lock.

v2 patch changes:
- introduced reset_files_struct() as Christoph Hellwig suggested

Found during OpenVZ stress testing.

Signed-Off-By: Pavel Emelianov <xemul@openvz.org>
Signed-Off-By: Kirill Korotaev <dev@openvz.org>