tomoyo: fix sparse warning

Fix sparse warning.

$ make C=2 SUBDIRS=security/tomoyo CF="-D__cold__="
 CHECK   security/tomoyo/common.c
 CHECK   security/tomoyo/realpath.c
 CHECK   security/tomoyo/tomoyo.c
security/tomoyo/tomoyo.c:110:8: warning: symbol 'buf' shadows an earlier one
security/tomoyo/tomoyo.c:100:7: originally declared here

Signed-off-by: Kentaro Takeda <takedakn@nttdata.co.jp>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Toshiharu Harada <haradats@nttdata.co.jp>
Signed-off-by: James Morris <jmorris@namei.org>