nilfs2: simplify nilfs_mdt_writeback()

The writepages() callback for metadata files, nilfs_mdt_writeback(),
calls nilfs_mdt_write_folio() for each dirty folio via writeback_iter(),
where read-only superblock checks, folio dirty state clearing on error,
and triggering of the log writer via nilfs_construct_segment() are
performed.  This helper function is used nowhere else.

Executing these operations per folio is inefficient:

- Checking if the filesystem has fallen back to read-only mode is
  repeated redundantly.

- Calling nilfs_construct_segment() inside the per-folio loop causes
  redundant log writer invocations for a single sync request, where only
  the first invocation performs actual log writing.

Unfold nilfs_mdt_write_folio() into nilfs_mdt_writeback() and consolidate
these operations at the function level.  The read-only check is moved to
the top of nilfs_mdt_writeback(), replacing per-folio
nilfs_clear_folio_dirty() calls with a single nilfs_clear_dirty_pages()
call to discard dirty pages across the mapping at once.  Additionally,
nilfs_construct_segment() is invoked once when 'sync_mode' is WB_SYNC_ALL
and the mapping is tagged dirty.

With these operations consolidated outside the loop, the remaining
per-folio actions (calling folio_redirty_for_writepage() on already
dirty folios and unlocking them) serve no purpose, so the folio iteration
itself is removed.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Viacheslav Dubeyko <slava@dubeyko.com>

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