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

Stop the HOSTFS filesystem from using iget() and read_inode().  Provide
hostfs_iget(), and call that instead of iget().  hostfs_iget() then uses
iget_locked() directly and returns a proper error code instead of an inode in
the event of an error.

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

Note that the contents of hostfs_kern.c need to be examined:

 (*) hostfs_iget() should perhaps subsume init_inode() and hostfs_read_inode().

 (*) It would appear that all hostfs inodes are the same inode because iget()
     was being called with inode number 0 - which forms the lookup key.

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Jeff Dike <jdike@addtoit.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>