btrfs: use correct count in btrfs_file_write_iter()

generic_write_checks() may modify iov_iter_count(), so we must get the
count after the call, not before. Using the wrong one has a couple of
consequences:

1. We check a longer range in check_can_nocow() for nowait than we're
   actually writing.
2. We create extra hole extent maps in btrfs_cont_expand(). As far as I
   can tell, this is harmless, but I might be missing something.

These issues are pretty minor, but let's fix it before something more
important trips on it.

Fixes: edf064e7 ("btrfs: nowait aio support")
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -1885,7 +1885,7 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 	bool sync = (file->f_flags & O_DSYNC) || IS_SYNC(file->f_mapping->host);
 	ssize_t err;
 	loff_t pos;
-	size_t count = iov_iter_count(from);
+	size_t count;
 	loff_t oldsize;
 	int clean_page = 0;
 
@@ -1906,6 +1906,7 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
 	}
 
 	pos = iocb->ki_pos;
+	count = iov_iter_count(from);
 	if (iocb->ki_flags & IOCB_NOWAIT) {
 		/*
 		 * We will allocate space in case nodatacow is not set,