depmod: Fix writing over array length

depmod: Fix writing over array length

Make sure return value in flush_stream_to() is the length written
if the value didn't the size. Fix warning on gcc 12.1:

	tools/depmod.c: In function ‘output_builtin_alias_bin’:
	tools/depmod.c:2465:24: warning: array subscript 4096 is above array bounds of ‘char[4096]’ [-Warray-bounds]
	 2465 |                 modname[len] = '\0';
	      |                 ~~~~~~~^~~~~
	tools/depmod.c:2460:22: note: while referencing ‘modname’
	 2460 |                 char modname[PATH_MAX];
	      |                      ^~~~~~~
	tools/depmod.c:2477:22: warning: array subscript 4096 is above array bounds of ‘char[4096]’ [-Warray-bounds]
	 2477 |                 value[len] = '\0';
	      |                 ~~~~~^~~~~
	tools/depmod.c:2461:22: note: while referencing ‘value’
	 2461 |                 char value[PATH_MAX];
	      |                      ^~~~~

Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>