Don't return EEXISTS from cache_inode_get

In the event that cache_inode_new_entry returns
CACHE_INODE_ENTRY_EXISTS, just return CACHE_INODE_SUCCESS.  This just
means someone raced us and installed the filehandle.  Since we have an
entry, it's usable.

Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>