ext4: Provide a splice-read wrapper

ext4: Provide a splice-read wrapper

Provide a splice_read wrapper for Ext4.  This does the inode shutdown check
before proceeding.  Splicing from DAX files and O_DIRECT fds is handled by
the caller.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Theodore Ts'o <tytso@mit.edu>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Jens Axboe <axboe@kernel.dk>
cc: Andreas Dilger <adilger.kernel@dilger.ca>
cc: linux-ext4@vger.kernel.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-mm@kvack.org
Link: https://lore.kernel.org/r/20230522135018.2742245-19-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>