btrfs: make inline extent read calculation much simpler

Currently we calculate inline extent read in a way that inline extent
can start at non-zero offset.

This is consistent with the inode selftests, which puts an inline extent
at file offset 5.

Meanwhile the inline extent creation code will only create inline extent
at file offset 0.

Furthermore with the introduction of tree-checker on file extents, we are
actively rejecting inline extent which starts at non-zero file offset.
And so far we haven't yet seen any report of rejected inline extents at
non-zero file offset.

This all means, the extra calculation to support inline extents at
non-zero file offset is mostly paper weight, and damaging the
readability of the code.

Thus this patch will:

- Add extra ASSERT()s to make sure involved file offset are all 0

- Remove @extent_offset calculation

- Simplify the involved code
  As several variables are now single-use, no need to declare them as
  a variable anymore.

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