The main purpose of mb cache is to achieve deduplication in
extended attributes. In use cases where opportunity for deduplication
is unlikely, it only adds overhead.

Add a mount option to explicitly turn off mb cache.
Suggested-by: NAndreas Dilger <adilger@dilger.ca>
Signed-off-by: NTahsin Erdogan <tahsin@google.com>
Signed-off-by: NTheodore Ts'o <tytso@mit.edu>