util/os_file: always use the 'grow' mechanism

Use fstat() only to pre-allocate a big enough buffer.

This fixes a race where if the file grows between fstat() and read()
we would be missing the end of the file, and if the file slims down
read() would just fail.

Fixes: 31696470 "util: add os_read_file() helper"
Reported-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 22c1657d)