ksmbd: fix wrong next length validation of ea buffer in smb2_set_ea()

There are multiple smb2_ea_info buffers in FILE_FULL_EA_INFORMATION request
from client. ksmbd find next smb2_ea_info using ->NextEntryOffset of
current smb2_ea_info. ksmbd need to validate buffer length Before
accessing the next ea. ksmbd should check buffer length using buf_len,
not next variable. next is the start offset of current ea that got from
previous ea.

Cc: stable@vger.kernel.org
Reported-by: zdi-disclosures@trendmicro.com # ZDI-CAN-21598
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>