Btrfs-progs: remove btrfs_init_path calls from ctree.c

Btrfs-progs: remove btrfs_init_path calls from ctree.c

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.

[Eric Sandeen: port kernel commit e00f730 to userspace]

(Note, the rest of userspace has an on-stack path, so the actual
function remains for now).

Signed-off-by: Chris Mason <chris.mason@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>