depmod: shut up gcc insufficinet buffer warning

depmod: shut up gcc insufficinet buffer warning

In a couple of places depmod concatenates the module directory and filename
with snprintf. This can technically overflow creating an unterminated string if
module directory name is long. Use openat instead as is done elsewhere in
depmod. This avoids the snprintf, the extra buffer on stack, and the gcc
warning. It may even fix a corner case when the module direcotry name is just
under PATH_MAX.

[ Lucas: fix up coding style and closing fd on error path ]

Signed-off-by: Michal Suchanek <msuchanek@suse.de>