btrfs: use num_extent_folios() in for loop bounds

btrfs: use num_extent_folios() in for loop bounds

As the helper num_extent_folios() is now __pure, we can use it in for
loop without storing its value in a variable explicitly, the compiler
will do this for us.

The effects on btrfs.ko is -200 bytes and there are stack space savings
too:

btrfs_clone_extent_buffer                               -8 (32 -> 24)
btrfs_clear_buffer_dirty                                -8 (48 -> 40)
clear_extent_buffer_uptodate                            -8 (40 -> 32)
set_extent_buffer_dirty                                 -8 (32 -> 24)
write_one_eb                                            -8 (88 -> 80)
set_extent_buffer_uptodate                              -8 (40 -> 32)
read_extent_buffer_pages_nowait                        -16 (64 -> 48)
find_extent_buffer                                      -8 (32 -> 24)

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