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 <nborisov@suse.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>