[RFC]fs: fsprivate data argument for get_block_t

[Dmitry Monakhov <dmonakhov@openvz.org>]

Hi, I'm wonder why generic get_block_t callback has not fsprivate
argument?
Each filesystem implement it's own get_block_t callback.
Each filesystem call generic functions which use callback from
known context, but where is no good way to pass that context 
to fs_specific_get_block callback.
This result in situation where fs-specific data passed via 
inode's data (xxx_inode->cur_get_block_data)
All this means that inode's private pointer becomes implicit
serialization point even if get_block() are allowed to be called
concurrently.

I'm going to add fsprivate argument, number of generic function which
use get_block_t is not really huge, so the patch should be too big.

If some one has any objections why this shouldn't be done, please
welcome.