kernel: sysctl: fix 'kbuf' uninitialized warning

- kernel/sysctl.c: In function ‘__do_proc_doulongvec_minmax’:
kernel/sysctl.c:1811:12: warning: ‘kbuf’ may be used uninitialized in
this function [-Wmaybe-uninitialized]
error, forbidden warning: sysctl.c:1811
scripts/Makefile.build:308: recipe for target 'kernel/sysctl.o' failed
make[1]: *** [kernel/sysctl.o] Error 1