boot/random-seed: create \loader\ dir if missing when seeding

boot/random-seed: create \loader\ dir if missing when seeding

When the random seed file doesn't exist but we have good entropy
(seeded_by_efi=true), we attempt to create it. This requires a handle
to the \loader\ directory, which may not exist on systems using
UKI+EFISTUB without systemd-boot installed.

Obtain the directory handle by first trying a read-only open; if that
returns EFI_NOT_FOUND, create the directory. We deliberately avoid
requesting write access on an already-present directory because some
firmware implementations return EFI_INVALID_PARAMETER for a
WRITE|CREATE open on an existing directory — this would be logged at
LOG_ERR and abort seed creation on systems where \loader\ exists but
random-seed does not (the normal systemd-boot layout).

Once a handle to \loader\ is obtained, open the seed file relative to
that handle rather than using the full path from root.

Introduced-by: c0e7046c17 ("boot: log about RO I/O errors at debug level.")
Fixes: #42801
Signed-off-by: dongshengyuan <dongshengyuan@uniontech.com>