ext4: do not block RWF_NOWAIT dio write on unallocated space

Since commit 378f32ba ("ext4: introduce direct I/O write using iomap
infrastructure") we don't properly bail out of RWF_NOWAIT direct IO
write if underlying blocks are not allocated. Also
ext4_dio_write_checks() does not honor RWF_NOWAIT when re-acquiring
i_rwsem. Fix both issues.

Fixes: 378f32ba ("ext4: introduce direct I/O write using iomap infrastructure")
Cc: stable@kernel.org
Reported-by: Filipe Manana <fdmanana@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200708153516.9507-1-jack@suse.czSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/file.c

@@ -427,6 +427,10 @@ static ssize_t ext4_dio_write_checks(struct kiocb *iocb, struct iov_iter *from,
 	 */
 	if (*ilock_shared && (!IS_NOSEC(inode) || *extend ||
 	     !ext4_overwrite_io(inode, offset, count))) {
+		if (iocb->ki_flags & IOCB_NOWAIT) {
+			ret = -EAGAIN;
+			goto out;
+		}
 		inode_unlock_shared(inode);
 		*ilock_shared = false;
 		inode_lock(inode);