btrfs: rename the functions to get inode and fs_info from an extent io tree

btrfs: rename the functions to get inode and fs_info from an extent io tree

These functions are exported so they should have a 'btrfs_' prefix by
convention, to make it clear they are btrfs specific and to avoid
collisions with functions from elsewhere in the kernel.

So add a 'btrfs_' prefix to their name to make it clear they are from
btrfs. Also remove the 'const' suffix from extent_io_tree_to_inode_const()
since there's no non-const variant anymore and makes the naming consistent
with extent_io_tree_to_fs_info() (no 'const' suffix and returns a const
pointer).

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>