insert-file-contents st_size cleanup

insert-file-contents st_size cleanup

* src/fileio.c (Finsert_file_contents): Store st_size,
not file_size_hint, into current_buffer->modtime_size,
since modtime_size is compared to a later st_size, not
to the actual file size.  Simplify buffer overflow detection
from file size, since this occurred only on regular files.

(cherry picked from commit 8d83402edc96b5a88096ae707241f4e8cb59691b)