btrfs: remove the pg_offset parameter from btrfs_get_extent()

The parameter @pg_offset of btrfs_get_extent() is only utilized for
inlined extent, and we already have an ASSERT() and tree-checker, to
make sure we can only get inline extent at file offset 0.

Any invalid inline extent with non-zero file offset would be rejected by
tree-checker in the first place.

Thus the @pg_offset parameter is not really necessary, just remove it.

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