loop-util: reopen device node if we shortcut loop device creation

loop-util: reopen device node if we shortcut loop device creation

The LoopDevice object supports a shortcut: if the backing fd we are
supposed to create a loopback device of refers to a
block device alrady then we'll use it as is – if we can – instead of
setting up an unnecessary loopback device that would be pretty much
the same as its backing device.

Previously, when doing this we'd just dup() the original backing fd and
use that. But that's problematic in case O_DIRECT was set on the fd,
since we'll keep that flag set on our copy too, which means we can't do
simple, regular IO on it anymore.

Thus, let's reopen the inode in this case with the exact access flags
we'd apply if we'd actually allocate and open a new loopback device.

Fixes: #21176