btrfs: make insert_inline_extent() accept one page directly

Since our inline extent cannot accept anything larger than a sector,
there is really no need to pass all the compressed pages to
insert_inline_extent().

And just in case, expand the ASSERT()s to make sure we only try inline
with compressed size no larger than sectorsize.

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