mount_masked_dir: errno from fs_move_mount_to() is discarded, fallback warning says "unknown error"

[eriksjolund]

When I discussed with Claude Opus 5 in #2170 , Claude Opus 5 mentioned 3 other problems. Here is one of them.

The problem described with my own words: The ternary operator condition in

crun/src/libcrun/linux.c

Line 1199 in 86e7e3e

warn_tmpfs_fallback_once (private_data, tmp_err ? tmp_err->msg : "unknown error");

always evaluates to false.

Claude Opus 5 thinks that errno from fs_move_mount_to() should not be discarded.

As title for this issue I used a suggestion from Claude Opus 5.