[JFFS2] fix sparse warnings in gc.c

fs/jffs2/gc.c:1147:29: warning: symbol 'jeb' shadows an earlier one
fs/jffs2/gc.c:1084:89: originally declared here
fs/jffs2/gc.c:1197:29: warning: symbol 'jeb' shadows an earlier one
fs/jffs2/gc.c:1084:89: originally declared here

Rename the unused 'jeb' argument to avoid this. We could potentially
remove the argument, but GCC should be doing that anyway.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>