btrfs: skip clearing EXTENT_DEFRAG for NOCOW ordered extents

btrfs: skip clearing EXTENT_DEFRAG for NOCOW ordered extents

In btrfs_finish_one_ordered(), clear_bits is unconditionally initialized
with EXTENT_DEFRAG.  For NOCOW ordered extents this is always a no-op
because should_nocow() already forces the COW path when EXTENT_DEFRAG is
set, so a NOCOW ordered extent can never have EXTENT_DEFRAG on its range.

Although harmless, the unconditional btrfs_clear_extent_bit() call still
performs a cold rbtree lookup under the io tree spinlock on every NOCOW
write completion.  Avoid this by only adding EXTENT_DEFRAG to clear_bits
for non-NOCOW ordered extents, and skip the call entirely when there are
no bits to clear.

Signed-off-by: Dave Chen <davechen@synology.com>
Signed-off-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>