ksmbd: fix incorrect handling of iterate_dir

if iterate_dir() returns non-negative value, caller has to treat it
as normal and check there is any error while populating dentry
information. ksmbd doesn't have to do anything because ksmbd already
checks too small OutputBufferLength to store one file information.

And because ctx->pos is set to file->f_pos when iterative_dir is called,
remove restart_ctx(). And if iterate_dir() return -EIO, which mean
directory entry is corrupted, return STATUS_FILE_CORRUPT_ERROR error
response.

This patch fixes some failure of SMB2_QUERY_DIRECTORY, which happens when
ntfs3 is local filesystem.

Fixes: e2f34481

 ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>