Make sendfile(2) more robust against file change.  This fixes a possible
crash when the file shrinks.  This also fixes sendfile(2) not sending more
data in a case when the file grows, and the request is open-ended or
specifies a size that is greater than old file size.

PR:           [[ https://bugs.freebsd.org/217789 | 217789 ]]
Reviewed by:  gallatin