um: Use correct data source in fpregs_legacy_set()

Read from the buffer pointed to by 'from' instead of '&buf', as
'buf' contains no valid data when 'ubuf' is NULL.

Fixes: b1e1bd2e ("um: Add helper functions to get/set state for SECCOMP")
Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Link: https://patch.msgid.link/20250606124428.148164-5-tiwei.btw@antgroup.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>