btrfs: extract the inline extent read code into its own function

Currently we have inline extent read code behind two levels of
indentation, factor them them out into a new function,
read_inline_extent(), to make it a little easier to read.

Since we're here, also remove @extent_offset and @pg_offset arguments
from uncompress_inline() function, as it's not possible to have inline
extents at non-inline file offset.

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