btrfs: use btrfs inodes in btrfs_rmdir() to avoid so much usage of BTRFS_I()

Almost everywhere we want to use a btrfs inode and therefore we have a
lot of calls to BTRFS_I(), making the code more verbose. Instead use btrfs
inode local variables to avoid so much use of BTRFS_I().

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>