s3: Plumb smb_filename around SMB_VFS_CHFLAGS

s3: Plumb smb_filename around SMB_VFS_CHFLAGS

SMB_VFS_CHFLAGS isn't actually getting the smb_filename struct for now
since it only operates on the basefile.  This is the strategy for all
path-based operations that will never actually operate on a stream.

By clarifying the meaning of path based operations that don't take an
smb_filename struct, modules that implement streams such as vfs_onefs
no longer need to implement SMB_VFS_CHFLAGS to ensure it's only called
on the base_name.