fs: btrfs: Introduce btrfs_read_extent_inline() and btrfs_read_extent_reg()

These two functions are used to do sector aligned read, which will be
later used to implement btrfs_file_read().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>