Inode lock not needed for ll_get_vino.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Concurrent ll_symlink (Client)

Unserialize simultaneous symlink and ll_symlink operations,
using the new Inode mutex.

This change includes updates to get_or_create and link, but
not their other callers (except for compilation).

Signed-off-by: Matt Benjamin <matt@linuxbox.com>