block: change default for discard and write zeroes to INT_MAX do not trim requests if the driver does not supply a limit through BlockLimits. For write zeroes we still keep a limit for the unsupported path to avoid allocating a big bounce buffer. Suggested-by: Kevin Wolf <kwolf@redhat.com> Suggested-by: Denis V. Lunev <den@openvz.org> Signed-off-by: Peter Lieven <pl@kamp.de> Signed-off-by: Kevin Wolf <kwolf@redhat.com>