cifs: define variable sized buffer for querydir responses

cifs: define variable sized buffer for querydir responses

QueryDirectory responses today are stored in one of two fixed
sized buffers: smallbuf (448 bytes) or bigbuf (16KB). These are
borrowed from server struct and are not sufficient for large-sized
query dir operations.

With this change we will now define a new buffer type specifically
for cifs_search_info to hold variable sized responses. These will
be allocated by kmalloc and freed by kfree.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>