dtlk: remove double call to nonseekable_open

dtlk_open currently has 2 calls to nonseekable_open which are both
executed on success path. It was not hurting to make the extra call as
nonseekable_open is only changing file->f_flags in idempotent way.
However the first nonseekable_open is indeed both unneeded and looks
suspicious.

The first nonseekable_open was added in 6244f13c51 ("Fix up a couple of
drivers - notable sg - for nonseekability."; 2004-Aug-7). The second
nonseekable_open call was introduced in dc5c724584 ("Remove ESPIPE logic
from drivers, letting the VFS layer handle it instead.; 2004-Aug-8). The
latter patch being mass change probably missed to remove
nonseekable_open that was introduced into dtlk_open the day before.

Fix it: remove the extra/unneeded nonseekable_open call and leave the
call to nonseekable_open only on the path where we are actually opening
the file.

Suggested-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Kirill Smelkov <kirr@nexedi.com>