gfs2: gfs2_inode_lookup rework

Rework gfs2_inode_lookup() to only set up the new inode's glocks after
verifying that the new inode is valid.

There is no need for flushing the inode glock work queue anymore now,
so remove that as well.

While at it, get rid of the useless wrapper around iget5_locked() and
its unnecessary is_bad_inode() check.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>