lindebugfs: Add debugfs_create_file_size()

This is the same as debugfs_create_file() but takes the initial size
of the file. In FreeBSD, the given size is ignored and
debugfs_create_file() is called.

Reviewed by: emaste, manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D37914

(cherry picked from commit a04aa80e773a4bfdc4e72bab85d6e16f0083745e)