BACKPORT: FROMLIST: ext4: implement speculative fault handling

We just need to make sure ext4_filemap_fault() doesn't block in the
speculative case as it is called with an rcu read lock held.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20210407014502.24091-32-michel@lespinasse.org/



Conflicts:
    fs/ext4/inode.c

1. The change in fs/ext4/inode.c is not needed since i_mmap_sem is not
used anymore.

Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: Idafc81074cf7f4b31985bdb24e0cc1597c91b875