libceph: fixup error handling in osdmap_apply_incremental()

libceph: fixup error handling in osdmap_apply_incremental()

The existing error handling scheme requires resetting err to -EINVAL
prior to calling any ceph_decode_* macro.  This is ugly and fragile,
and there already are a few places where we would return 0 on error,
due to a missing reset.  Follow osdmap_decode() and fix this by adding
a special e_inval label to be used by all ceph_decode_* macros.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Alex Elder <elder@linaro.org>