cifs: Validate content of NFS reparse point buffer

Symlink target location stored in DataBuffer is encoded in UTF-16. So check
that symlink DataBuffer length is non-zero and even number. And check that
DataBuffer does not contain UTF-16 null codepoint because Linux cannot
process symlink with null byte.

DataBuffer for char and block devices is 8 bytes long as it contains two
32-bit numbers (major and minor). Add check for this.

DataBuffer buffer for sockets and fifos zero-length. Add checks for this.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>