pstore/blk: Use the normal block device I/O path

Stop poking into block layer internals and just open the block device
file an use kernel_read and kernel_write on it. Note that this means
the transformation from name_to_dev_t can't be used anymore when
pstore_blk is loaded as a module: a full filesystem device path name
must be used instead. Additionally removes ":internal:" kerndoc link,
since no such documentation remains.
Co-developed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kees Cook <keescook@chromium.org>

Documentation/admin-guide/pstore-blk.rst

@@ -227,8 +227,5 @@ For developer reference, here are all the important structures and APIs:
 .. kernel-doc:: include/linux/pstore_zone.h
    :internal:
 
-.. kernel-doc:: fs/pstore/blk.c
-   :internal:
-
 .. kernel-doc:: include/linux/pstore_blk.h
    :internal: