eCryptfs: Introduce persistent lower files for each eCryptfs inode

Currently, eCryptfs directly accesses the lower inode address space, doing things like grab_cache_page() on lower_inode->i_mapping. It really should not do that. The main point of this patch set is to make all I/O with the lower files go through vfs_read() and vfs_write() instead.

In order to accomplish this, eCryptfs needs a way to call vfs_read() and vfs_write() on the lower file when ecryptfs_aops->readpage() and ecryptfs_aops->writepage() are called. I propose keeping a persistent lower file around for each eCryptfs inode. This is the only lower file that eCryptfs will open for any given eCryptfs inode; multiple eCryptfs files may map to this one persistent lower file. When the eCrypfs inode is destroyed, this persistent lower file is closed.

Consolidating all reads and writes to the lower file to a single execution path simplifies the code. This should also make it easier to port eCryptfs to use the asynchronous crypto API functions. Note that this patch set also removes all direct calls to lower prepare_write() and commite_write(), fixing an oops when mounted on NFS.

Mike

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