file-posix: Prepare permission code for fd switching

file-posix: Prepare permission code for fd switching

In order to be able to dynamically reopen the file read-only or
read-write, depending on the users that are attached, we need to be able
to switch to a different file descriptor during the permission change.

This interacts with reopen, which also creates a new file descriptor and
performs permission changes internally. In this case, the permission
change code must reuse the reopen file descriptor instead of creating a
third one.

In turn, reopen can drop its code to copy file locks to the new file
descriptor because that is now done when applying the new permissions.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>