b=19808 fix d_obtain_alias() misuse due to compat macro.

b=19808 fix d_obtain_alias() misuse due to compat macro.

o=Adreas.Dilger
i=yangsheng
i=Oleg.Drokin

ll_iget_for_nfs() get a change from d_alloc_anon() to d_obtain_alias().
The former would NOT release the inode reference if there was an error, while
the latter does.  Hide this detail in the d_obtain_alias() compat macro, for
kernels that do not have this function, and remove it from the error handling
code at the caller, where it would drop the inode reference twice in rare error
cases on all kernels WITH d_obtain_alias() support (RHEL5 and newer).