rbd: don't assert on writes to snapshots

rbd: don't assert on writes to snapshots

The check added in commit 721c7fc701c7 ("block: fail op_is_write()
requests to read-only partitions") was lifted in commit a32e236eb93e
("Partially revert "block: fail op_is_write() requests to read-only
partitions"").  Basic things like user triggered writes and discards
are still caught, but internal kernel users can submit anything.  In
particular, ext4 will attempt to write to the superblock if it detects
errors in the filesystem, even if the filesystem is mounted read-only
on a read-only partition.

The assert is overkill regardless.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>