cifs: fix artificial limit on reading symlinks

There's no reason to limit the size of a symlink that we can read to
4000 bytes. That may be nowhere near PATH_MAX if the server is sending
UCS2 strings. CIFS should be able to read in a symlink up to the size of
the buffer. The size of the header has already been accounted for when
creating the slabcache, so CIFSMaxBufSize should be the correct size to
pass in.

Fixes samba bug #6384.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>