* filer: skip UpdateEntry when inline content is unchanged
SaveInsideFiler rewrites config files (IAM identities, filer.conf,
remote mappings, policies) unconditionally. Each no-op UpdateEntry is a
metadata event the local meta log persists to /topics/.system/log,
which appends a chunk to a volume. A client that rewrites identical
config on a timer, e.g. the seaweedfs-operator 5-minute resync calling
UpdateUser with unchanged actions, keeps .dat/.idx files growing on an
otherwise idle cluster and prevents HDD spindown
(seaweedfs/seaweedfs#11571).
Skip the UpdateEntry when the stored inline content is byte-identical,
so unchanged writes produce no metadata event and no volume writes.
* filer: test that identical SaveInsideFiler writes skip UpdateEntry
* filer: require stamped Md5 before skipping identical writes
An entry holding identical content but no Md5 (written before hashing,
or by a tool that cleared it) would never get the stamp that
IF_ETAG_MATCH conditional writes key off. Skip only when both the
stored content and its Md5 match, so one write still lands to repair
the stamp.
