btrfs: fix incomplete parameter rename in btrfs_decompress()

btrfs: fix incomplete parameter rename in btrfs_decompress()

Commit 2c25716dcc25 ("btrfs: zlib: fix and simplify the inline extent
decompression") renamed the 'start_byte' parameter to 'dest_pgoff' in
the btrfs_decompress(). The remaining 'start_byte' references are
inconsistent with the actual implementation and may cause confusion for
developers.

Ensure consistency between function declaration and implementation.

Signed-off-by: Zhen Ni <zhen.ni@easystack.cn>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>