btrfs: move inode prototypes to btrfs_inode.h

I initially wanted to make a new header file for this, but these
prototypes do naturally fit into btrfs_inode.h.  If we want to extract
vfs from pure btrfs code in the future we may need to split this up, but
btrfs_inode embeds the vfs_inode, so it makes sense to put the
prototypes in this header for now.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>