btrfs: tests: prepare extent map tests for strict alignment checks

btrfs: tests: prepare extent map tests for strict alignment checks

Currently the extent map self tests have the following points that will
cause false alerts for the incoming strict extent map alignment checks:

- Incorrect inlined extent map size

  Which is not following what the kernel is doing for inlined extents,
  as btrfs_extent_item_to_extent_map() always uses the fs block size as
  the length, not the ram_bytes.

  Fix it by using SZ_4K as extent map's length.

- Incorrect btrfs_fs_info::sectorsize

  As we always use PAGE_SIZE, which can be values larger than 4K.
  Meanwhile all the immediate numbers used are based on 4K fs block size
  in the test case.

  Fix it by using fixed SZ_4K fs block size when allocating the dummy
  btrfs_fs_info.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>