UBIFS: fix sparse warnings

fs/ubifs/compress.c:111:8: warning: incorrect type in argument 5 (different signedness)
fs/ubifs/compress.c:111:8:    expected unsigned int *dlen
fs/ubifs/compress.c:111:8:    got int *out_len
fs/ubifs/compress.c:175:10: warning: incorrect type in argument 5 (different signedness)
fs/ubifs/compress.c:175:10:    expected unsigned int *dlen
fs/ubifs/compress.c:175:10:    got int *out_len

Fix this by adding a cast to (unsigned int *). We guarantee that
our lengths are small and no overflow is possible.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>