convert vfs_dedupe_file_range().

fdput() is followed by checking fatal_signal_pending() (and aborting
the loop in such case).  fdput() is transposable with that check.
Yes, it'll probably end up with slightly fatter code (call after the
check has returned false + call on the almost never taken out-of-line path
instead of one call before the check), but it's not worth bothering with
explicit extra scope there (or dragging the check into the loop condition,
for that matter).

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>