LU-12799 ptlrpc: return proper error code

LU-12799 ptlrpc: return proper error code

from ptlrpc_disconnect_prep_req() using ERR_PTR()
as the callers expect.

Lustre-change: https://review.whamcloud.com/36282
Lustre-commit: 9e2620d75cce1e1b4855704ddd9a994ce8e8d650

Fixes: 5a6ceb664f07 ("LU-7236 ptlrpc: idle connections can disconnect")
Change-Id: I5493194a1f18f3d0b559921b7859bf835585ba58
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-on: https://review.whamcloud.com/37164
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>