Don't use d_alloc_anon for open_by_handle

[xaiki@xxxxxxx]

One of our DMAPI user found that in some cases inodes weren't getting through
xfs_inactive() in any reasonable amount of time. Investigation tracked it down
to the use of d_alloc_anon() combined with another thread accessing the same
inode via an open().

So we introduce a stripped down version of d_alloc_anon, that won't try to find
an existing dentry, nor will hash it.