fs: call security_d_instantiate in d_obtain_alias V2

While trying to track down some NFS problems with BTRFS, I kept noticing I was
getting -EACCESS for no apparent reason.  Eric Paris and printk() helped me
figure out that it was SELinux that was giving me grief, with the following
denial

type=AVC msg=audit(1290013638.413:95): avc:  denied  { 0x800000 } for  pid=1772
comm="nfsd" name="" dev=sda1 ino=256 scontext=system_u:system_r:kernel_t:s0
tcontext=system_u:object_r:unlabeled_t:s0 tclass=file

Turns out this is because in d_obtain_alias if we can't find an alias we create
one and do all the normal instantiation stuff, but we don't do the
security_d_instantiate.

Usually we are protected from getting a hashed dentry that hasn't yet run
security_d_instantiate() by the parent's i_mutex, but obviously this isn't an
option there, so in order to deal with the case that a second thread comes in
and finds our new dentry before we get to run security_d_instantiate(), we go
ahead and call it if we find a dentry already.  Eric assures me that this is ok
as the code checks to see if the dentry has been initialized already so calling
security_d_instantiate() against the same dentry multiple times is ok.  With
this patch I'm no longer getting errant -EACCESS values.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>