x86/xstate: Fix latent bugs in compress_xsave_states()

x86/xstate: Fix latent bugs in compress_xsave_states()

compress_xsave_states() mustn't read xstate_bv or xcomp_bv before first
confirming that the input buffer is large enough.  It also doesn't cope with
compressed input.  Make all of these problems the callers responsbility to
ensure.

Simplify the decompression logic by inlining get_xsave_addr().  As xstate_bv
is previously validated, dest won't ever been NULL.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>