fs: aio ioprio add explicit block layer dependence

Previously, the ioprio_check_cap function was only defined when CONFIG_BLOCK
was set. Make this relationship explicit and add a stub for !CONFIG_BLOCK.

Signed-off-by: Adam Manzanares <adam.manzanares@wdc.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>