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 --git a/fs/read_write.c b/fs/read_write.c
index 31c6efa..d29d2a3 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -396,7 +396,7 @@
 
 	if (!(file->f_mode & FMODE_READ))
 		return -EBADF;
-	if (!file->f_op->read && !file->f_op->aio_read)
+	if (!(file->f_mode & FMODE_CAN_READ))
 		return -EINVAL;
 	if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
 		return -EFAULT;
@@ -445,7 +445,7 @@
 	const char __user *p;
 	ssize_t ret;
 
-	if (!file->f_op->write && !file->f_op->aio_write)
+	if (!(file->f_mode & FMODE_CAN_WRITE))
 		return -EINVAL;
 
 	old_fs = get_fs();
@@ -472,7 +472,7 @@
 
 	if (!(file->f_mode & FMODE_WRITE))
 		return -EBADF;
-	if (!file->f_op->write && !file->f_op->aio_write)
+	if (!(file->f_mode & FMODE_CAN_WRITE))
 		return -EINVAL;
 	if (unlikely(!access_ok(VERIFY_READ, buf, count)))
 		return -EFAULT;
@@ -785,7 +785,7 @@
 {
 	if (!(file->f_mode & FMODE_READ))
 		return -EBADF;
-	if (!file->f_op->aio_read && !file->f_op->read)
+	if (!(file->f_mode & FMODE_CAN_READ))
 		return -EINVAL;
 
 	return do_readv_writev(READ, file, vec, vlen, pos);
@@ -798,7 +798,7 @@
 {
 	if (!(file->f_mode & FMODE_WRITE))
 		return -EBADF;
-	if (!file->f_op->aio_write && !file->f_op->write)
+	if (!(file->f_mode & FMODE_CAN_WRITE))
 		return -EINVAL;
 
 	return do_readv_writev(WRITE, file, vec, vlen, pos);
@@ -964,7 +964,7 @@
 		goto out;
 
 	ret = -EINVAL;
-	if (!file->f_op->aio_read && !file->f_op->read)
+	if (!(file->f_mode & FMODE_CAN_READ))
 		goto out;
 
 	ret = compat_do_readv_writev(READ, file, vec, vlen, pos);
@@ -1041,7 +1041,7 @@
 		goto out;
 
 	ret = -EINVAL;
-	if (!file->f_op->aio_write && !file->f_op->write)
+	if (!(file->f_mode & FMODE_CAN_WRITE))
 		goto out;
 
 	ret = compat_do_readv_writev(WRITE, file, vec, vlen, pos);