btrfs: remove unnecessary memory barrier in btrfs_direct_IO

Commit 38851cc19a ("Btrfs: implement unlocked dio write") implemented
unlocked dio write, allowing multiple dio writers to write to
non-overlapping, and non-eof-extending regions. In doing so it also
introduced a broken memory barrier. It is broken due to 2 things:

1. Memory barriers _MUST_ always be paired, this is clearly not the case
   here

2. Checkpatch actually produces a warning if a memory barrier is
   introduced that doesn't have a comment explaining how it's being
   paired.

Specifically for inode::i_dio_count that's wrapped inside
inode_dio_begin, there is no explicit barrier semantics attached, so
removing is fine as the atomic is used in common the waiter/wakeup
pattern.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
[ enhance changelog ]
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -8815,7 +8815,6 @@ static ssize_t btrfs_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
 		return 0;
 
 	inode_dio_begin(inode);
-	smp_mb__after_atomic();
 
 	/*
 	 * The generic stuff only does filemap_write_and_wait_range, which