ceph: add read/modify/write to ceph_sync_write

When doing a synchronous write on an encrypted inode, we have no
guarantee that the caller is writing crypto block-aligned data. When
that happens, we must do a read/modify/write cycle.

First, expand the range to cover complete blocks. If we had to change
the original pos or length, issue a read to fill the first and/or last
pages, and fetch the version of the object from the result.

We then copy data into the pages as usual, encrypt the result and issue
a write prefixed by an assertion that the version hasn't changed. If it has
changed then we restart the whole thing again.

If there is no object at that position in the file (-ENOENT), we prefix
the write on an exclusive create of the object instead.
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>