Btrfs: remove btrfs_init_path

btrfs_init_path was initially used when the path objects were on the
stack.  Now all the work is done by btrfs_alloc_path and btrfs_init_path
isn't required.

This patch removes it, and just uses kmem_cache_zalloc to zero out the object.

Signed-off-by: Chris Mason <chris.mason@oracle.com>