ext2: remove useless call to brelse() in ext2_free_inode()

This patch removes a useless call to brelse(bitmap_bh) since at that
point bitmap_bh is NULL and slightly cleans up bitmap_bh handling.

Signed-off-by: Francis Moreau <francis.moro@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>