find-esp: rework verify_fsroot_dir() around statx()

find-esp: rework verify_fsroot_dir() around statx()

Let's rework verify_fsroot_dir() around two statx() calls, which should
usually give us all information we need. Let's then add a call to the
btrfs ioctl only if the caller actually wants to know the backing block
device (which it generally only wants if it then intends to verify
partition type, and similar).

This makes things more robust on new kernels, and bypasses the btrfs
ioctls in environments where we cannot actually interpret the results
properly (since they reference btrfs devices via /dev/ paths, which we
likely cannot resolve in containers)

Fixes: #24616