fs: avoid fdput() after failed fdget() in vfs_dedupe_file_range()

It's a fairly inconsequential bug, since fdput() won't actually try to
fput() the file due to fd.flags (and thus FDPUT_FPUT) being zero in
the failure case, but most other vfs code takes steps to avoid this.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>