s3:smbd: take less than SMB_BUFFER_SIZE_MIN ('500') as header overhead in ipc.c

s3:smbd: take less than SMB_BUFFER_SIZE_MIN ('500') as header overhead in ipc.c

We're now sure that sconn->smb1.sessions.max_send is >= SMB_BUFFER_SIZE_MIN.
in order to garantee some progress we need to make sure our assumed
header overhead is less than SMB_BUFFER_SIZE_MIN.

Assuming 372 bytes for the SMBtrans headers should still be more than
enough.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10422
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>