fs/adfs: bigdir: Fix an error code in adfs_fplus_read()

This code accidentally returns success, but it should return the
-EIO error code from adfs_fplus_validate_header().

Acked-by: Russell King <rmk+kernel@armlinux.org.uk>
Fixes: d79288b4 ("fs/adfs: bigdir: calculate and validate directory checkbyte")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>