Unionfs: stop using iget() and read_inode()

Unionfs: stop using iget() and read_inode()

Replace unionfs_read_inode() with unionfs_iget(), and call that instead of
iget().  unionfs_iget() then uses iget_locked() directly and returns a
proper error code instead of an inode in the event of an error.

unionfs_fill_super() returns any error incurred when getting the root inode
instead of EINVAL.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Erez Zadok <ezk@cs.sunysb.edu>