ksmbd: remove smb2_buf_length in smb2_hdr

mainline inclusion
from mainline-5.16-rc1
commit cb451720
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I60T7G
CVE: NA

Reference: https://git.kernel.org/torvalds/linux/c/cb4517201b8a



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To move smb2_hdr to smbfs_common, This patch remove smb2_buf_length
variable in smb2_hdr. Also, declare smb2_get_msg function to get smb2
request/response from ->request/response_buf.

Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zhong Jinghua <zhongjinghua@huawei.com>