jffs2: Fix long-standing bug with symlink garbage collection.

Ever since jffs2_garbage_collect_metadata() was first half-written in
February 2001, it's been broken on architectures where 'char' is signed.
When garbage collecting a symlink with target length above 127, the payload
length would end up negative, causing interesting and bad things to happen.

Cc: stable@kernel.org
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>