kclient: setattr on inode (not path) if holding WR cap

kclient: setattr on inode (not path) if holding WR cap

For newly created files, this avoids contention in the MDS over the dentry
xlock (e.g., for an untar).

It is slightly risky, because we could race with a caps release and MDS
cache trim.  Pretty unlikely, however, because cap release would also
involve MDS journaling.