fix filler_t callback type mismatches v2

[Christoph Hellwig <hch@xxxxxx>]

Casting mapping->a_ops->readpage to filler_t causes an indirect call
type mismatch with Control-Flow Integrity checking. This change fixes
the mismatch in read_cache_page_gfp and read_mapping_page by adding
using a NULL filler argument as an indication to call ->readpage
directly, and by passing the right parameter callbacks in nfs and jffs2.

Changes since v1:
 - add the 9p patch to the series
 - drop the nfs patch that has been merged

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