ksmbd: move oplock handling after unlock parent dir

ksmbd should process secound parallel smb2 create request during waiting
oplock break ack. parent lock range that is too large in smb2_open() causes
smb2_open() to be serialized. Move the oplock handling to the bottom of
smb2_open() and make it called after parent unlock. This fixes the failure
of smb2.lease.breaking1 testcase.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>