tail: fix "tail -F" file rotation detection

When checking if the newly opened file is the same as the old one,
we need to fstat() the new file descriptor, not the old one again.

Reviewed by: glebius
Sponsored by: Netflix

(cherry picked from commit 7787e7eed9d2a43fb6eb66666040f1b495995a2f)
(cherry picked from commit 8c59e863e2c03c3b4ab2c403243561a4cc4fcb10)

Approved by: so
Errata: FreeBSD-EN-22:05.tail