[#970] fstree: Add build tag to enable generic version on linux

Unless tested, generic version can start gaining bugs. With a separate
build tag we can have the best of both worlds:
1. Use optimized implementation for linux by default.
2. Run tests or benchmarks for both. Note that they are not actually
   run automatically now, but this is at leas possible.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/local_object_storage/blobstor/fstree/fstree_write_linux.go

@@ -1,4 +1,4 @@
-//go:build linux
+//go:build linux && !fstree_generic
 
 package fstree
 

pkg/local_object_storage/blobstor/fstree/fstree_write_specific.go

@@ -1,4 +1,4 @@
-//go:build !linux
+//go:build !linux || fstree_generic
 
 package fstree