pipe: make F_{GET,SET}PIPE_SZ deal with byte sizes

Instead of requiring an exact number of pages as the argument and
return value, change the API to deal with number of bytes instead.

This also relaxes the requirement that the passed in size must
result in a power-of-2 page array size. Round up to the nearest
power-of-2 automatically and return the resulting size of the pipe
on success.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>