export operations rewrite

This patchset is a medium scale rewrite of the export operations interface. The goal is to make the interface less complex, and easier to understand from the filesystem side, aswell as preparing generic support for exporting of 64bit inode numbers.

This touches all nfs exporting filesystems, and I've done testing on all of the filesystems I have here locally (xfs, ext2, ext3, reiserfs, jfs)

Compared to the last version posted earlier this year I've moved the generic fh_to_dentry and generic_fh_to_parent method to libfs.c so that the filesystems don't have to pull in the exportfs module but only nfsd has to. That means I have to expose that ugly exportfs_d_alloc helper, but I have plans to get rid of that one entirely by merging it with d_alloc_anon. More on that in another patch series.

Note that this patch series is against mainline. There will be some xfs changes landing in -mm soon that revamp lots of the code touched here. They should hopefully include the first path in the series so it can be simply dropped, but the xfs conversion will need some smaller updates. I will send this update as soon as the xfs tree updates get pulled into -mm.

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