block-backend: convert blk_foo wrappers to use int64_t bytes parameter

Convert blk_pdiscard, blk_pwrite_compressed, blk_pwrite_zeroes.
These are just wrappers for functions with int64_t argument, so allow
passing int64_t as well. Parameter type becomes wider so all callers
should be OK with it.

Note that requests exceeding INT_MAX are still restricted by
blk_check_byte_request().

Note also that we don't (and are not going to) convert blk_pwrite and
blk_pread: these functions return number of bytes on success, so to
update them, we should change return type to int64_t as well, which
will lead to investigating and updating all callers which is too much.

So, blk_pread and blk_pwrite remain unchanged.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211006131718.214235-9-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweaks]
Signed-off-by: Eric Blake <eblake@redhat.com>