btrfs: check snapshot_force_cow earlier in can_nocow_file_extent()

btrfs: check snapshot_force_cow earlier in can_nocow_file_extent()

When a snapshot is being created, the atomic counter snapshot_force_cow
is incremented to force incoming writes to fallback to COW. This is a
critical mechanism to protect the consistency of the snapshot being taken.

Currently, can_nocow_file_extent() checks this counter only after
performing several checks, most notably the expensive cross-reference
check via btrfs_cross_ref_exist(). btrfs_cross_ref_exist() releases the
path and performs a search in the extent tree or backref cache, which
involves btree traversals and locking overhead.

Moves the snapshot_force_cow check to the very beginning of
can_nocow_file_extent().

This reordering is safe and beneficial because:

1. args->writeback_path is invariant for the duration of the call (set
   by caller run_delalloc_nocow).

2. is_freespace_inode is a static property of the inode.

3. The state of snapshot_force_cow is driven by the btrfs_mksnapshot()
   process. Checking it earlier does not change the outcome of the NOCOW
   decision, but effectively prunes the expensive code path when a
   fallback to COW is inevitable.

By failing fast when a snapshot is pending, we avoid the unnecessary
overhead of btrfs_cross_ref_exist() and other extent item checks in the
scenario where NOCOW is already known to be impossible.

Signed-off-by: Chen Guan Jie <jk.chen1095@gmail.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>