recvtftp() is broken for large files, report file size

The tftp download for large files will cause internal block id
to wrap to 0 as the data type is unsigned short.

Also provide file size information for stat.

PR: 200500
Reported by: tsoome
Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D7660