tools lib api: Fix filename__write_int() writing uninitialized stack data

tools lib api: Fix filename__write_int() writing uninitialized stack data

filename__write_int() formats an integer into a 64-byte buffer with
sprintf() then passes sizeof(buf) (64) as the write length.  This
writes all 64 bytes including uninitialized stack data past the
formatted string.  Most sysfs files reject the oversized write,
making the function always return -1.

Fix by capturing the sprintf() return value and using it as the
write length.

Reported-by: sashiko-bot <sashiko-bot@kernel.org>
Fixes: 3b00ea938653d136 ("tools lib api fs: Add sysfs__write_int function")
Cc: Kan Liang <kan.liang@intel.com>
Assisted-by: Claude:claude-opus-4.6
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>