ksmbd: uninterruptible wait for a file being unlocked

the wait can be canceled by SMB2_CANCEL, SMB2_CLOSE,
SMB2_LOGOFF, disconnection or shutdown, we don't have
to use wait_event_interruptible.

And this remove the warning from Coverity:

CID 1502834 (#1 of 1): Unused value (UNUSED_VALUE)
returned_value: Assigning value from ksmbd_vfs_posix_lock_wait(flock)
to err here, but that stored value is overwritten before it can be used.

Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>