orangefs: validate getxattr response length

orangefs: validate getxattr response length

orangefs_inode_getxattr() trusts the userspace-client-controlled
downcall.resp.getxattr.val_sz and uses it as a memcpy() length
both for the temporary user buffer and the cached xattr buffer.
Reject malformed negative or oversized lengths before copying
response bytes.

Reported-by: Hyungjung Joo <jhj140711@gmail.com>
Signed-off-by: HyungJung Joo <jhj140711@gmail.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>