fd-util: rename fds_are_same_mount() fds_inode_and_mount_same()

fd-util: rename fds_are_same_mount() fds_inode_and_mount_same()

The old name suggested this would only check if the two inodes are on
the same mount, but it actually checks if they are the same inodes too.

Let's rename it to make this clearer, in particular as we have
both statx_inode_same() and statx_mount_same() already, and they are
even used here, and hence very confusing.

This also drops two checks from the test case, which are simply wrong.
Given they apparently weren't load bearing (since no CI tripped up),
let's just drop them.