buffer: eliminate the need to call free_more_memory() in __getblk_slow()

buffer: eliminate the need to call free_more_memory() in __getblk_slow()

Since the previous commit removed any case where grow_buffers()
would return failure due to memory allocations, we can safely
remove the case where we have to call free_more_memory() in
this function.

Since this is also the last user of free_more_memory(), kill
it off completely.

Reviewed-by: Nikolay Borisov <[email protected]>
Reviewed-by: Jan Kara <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>