tmpfs: security xattr setting on inode creation

Adds to generic xattr support introduced in Linux 3.0 by implementing
initxattrs callback.  This enables consulting of security attributes from
LSM and EVM when inode is created.

[hughd@google.com: moved under CONFIG_TMPFS_XATTR, with memcpy in shmem_xattr_alloc]
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@intel.com>
Reviewed-by: James Morris <james.l.morris@oracle.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>