btrfs: allow idmapped mount

Now that we converted btrfs internally to account for idmapped mounts
allow the creation of idmapped mounts on by setting the FS_ALLOW_IDMAP
flag.  We only need to raise this flag on the btrfs_root_fs_type
filesystem since btrfs_mount_root() is ultimately responsible for
allocating the superblock and is called into from btrfs_mount()
associated with btrfs_fs_type.

The conversion of the btrfs inode operations was straightforward.
Regarding btrfs specific ioctls that perform checks based on inode
permissions only those have been allowed that are not filesystem wide
operations and hence can be reasonably charged against a specific mount.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>