ext4: skip orphan cleanup if fs has unknown ROCOMPAT features

Orphan cleanup is currently executed even if the file system has some
number of unknown ROCOMPAT features, which deletes inodes and frees
blocks, which could be very bad for some RO_COMPAT features,
especially the SNAPSHOT feature.

This patch skips the orphan cleanup if it contains readonly compatible
features not known by this ext4 implementation, which would prevent
the fs from being mounted (or remounted) readwrite.

Signed-off-by: Amir Goldstein <amir73il@users.sf.net>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>