fs: create file_readable() and file_writable() functions

Create functions to simplify if file_ops contain either a read
or aio_read op, or likewise write or aio_write. We will be adding
read_iter and write_iter and don't need to be complicating the code
in multiple places.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Signed-off-by: zwliew <zhaoweiliew@gmail.com>

Conflicts:
	include/linux/fs.h

Change-Id: Ib91d69da5e370ecdd37fecad803e4be4fa0d53e6