smb: client: improve logic in allocate_mr_list()

- use 'mr' as variable name
- use goto lables for easier cleanup
- use destroy_mr_list()
- style fixes
- INIT_WORK(&sc->mr_io.recovery_work, smbd_mr_recovery_work) on success

This will make further changes easier.

Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>