btrfs: update stale comment for log_new_dir_dentries()

The comment refers to the function log_dir_items() in order to check why
the inodes of new directory entries need to be logged, but the relevant
comments are no longer at log_dir_items(), they were moved to the function
process_dir_items_leaf() in commit eb10d85e ("btrfs: factor out the
copying loop of dir items from log_dir_items()"). So update it with the
current function name.

Also remove references with i_mutex to "VFS lock", since the inode lock
is no longer a mutex since 2016 (it's now a rw semaphore).

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>