CIFS: remove an unneeded NULL check

Smatch complains because we dereference "ses->server" without checking
some lines earlier inside the call to get_next_mid(ses->server).

	fs/cifs/cifssmb.c:4921 CIFSGetDFSRefer()
	warn: variable dereferenced before check 'ses->server' (see line 4899)

There is only one caller for this function get_dfs_path() and it always
passes a non-null "ses->server" pointer so this NULL check can be
removed.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steve French <smfrench@gmail.com>