dissect-image: fix wrong errno on pread failure

dissect-image: fix wrong errno on pread failure

In acquire_sig_for_roothash() the pread() failure branch returns
-ENOMEM, which is copy-pasted from the malloc() check immediately
above it:

    _cleanup_free_ char *buf = new(char, partition_size+1);
    if (!buf)
            return -ENOMEM;

    ssize_t n = pread(fd, buf, partition_size, partition_offset);
    if (n < 0)
            return -ENOMEM;

pread() sets errno to the actual I/O failure (EIO, EINTR, EBADF,
...). Aliasing those to -ENOMEM misleads dissect_log_error() and
any caller that branches on the returned code; verity signature
lookup failures get reported as out-of-memory.

Fix by returning -errno from the pread() failure branch. The
malloc() branch above is correct and unchanged.

Fixes: 98ca65c36aa9 ("dissect: check that roothash in signature matches before selecting partition")
Assisted-by: kres (claude-opus-4-7)
Signed-off-by: Chris Mason <clm@meta.com>