CIFS: remove local xattr definitions

Local XATTR_TRUSTED_PREFIX_LEN and XATTR_SECURITY_PREFIX_LEN definitions
redefined ones in 'linux/xattr.h'. This was caused by commit 9d8f13ba
("security: new security_inode_init_security API adds function callback")
including 'linux/xattr.h' in 'linux/security.h'.

In file included from include/linux/security.h:39,
                 from include/net/sock.h:54,
                 from fs/cifs/cifspdu.h:25,
                 from fs/cifs/xattr.c:26:

This patch removes the local definitions.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>