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: b1e1bd2 ("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>

Author: btw616

arch/x86/um/ptrace.c

@@ -161,7 +161,7 @@ static int fpregs_legacy_set(struct task_struct *target,
 		from = kbuf;
 	}
 
-	return um_fxsr_from_i387(fxsave, &buf);
+	return um_fxsr_from_i387(fxsave, from);
 }
 #endif