smb: client: reduce stack usage in smb_send_rqst()

Clang warns about exceeded stack frame size

  fs/smb/client/transport.c:420:1: warning: stack frame size (1048)
  exceeds limit (1024) in 'smb_send_rqst' [-Wframe-larger-than]

Fix this by allocating a structure that will hold transform header and
compound requests.

Signed-off-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>