iget: stop ROMFS from using iget() and read_inode()

Stop the ROMFS filesystem from using iget() and read_inode().  Replace
romfs_read_inode() with romfs_iget(), and call that instead of iget().
romfs_iget() then uses iget_locked() directly and returns a proper error code
instead of an inode in the event of an error.

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

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>