replace checking for ->read/->aio_read presence with check in ->f_mode

Since we are about to introduce new methods (read_iter/write_iter), the
tests in a bunch of places would have to grow inconveniently.  Check
once (at open() time) and store results in ->f_mode as FMODE_CAN_READ
and FMODE_CAN_WRITE resp.  It might end up being a temporary measure -
once everything switches from ->aio_{read,write} to ->{read,write}_iter
it might make sense to return to open-coded checks.  We'll see...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --cc drivers/mtd/nand/nandsim.c
Simple merge

diff --cc drivers/usb/gadget/storage_common.c
Simple merge

diff --cc fs/file_table.c
Simple merge

diff --cc fs/open.c
Simple merge

diff --cc fs/read_write.c
Simple merge

diff --cc include/linux/fs.h
Simple merge