ceph: don't allow copy_file_range when stripe_count != 1

copy_file_range tries to use the OSD 'copy-from' operation, which simply
performs a full object copy.  Unfortunately, the implementation of this
system call assumes that stripe_count is always set to 1 and doesn't take
into account that the data may be striped across an object set.  If the
file layout has stripe_count different from 1, then the destination file
data will be corrupted.

For example:

Consider a 8 MiB file with 4 MiB object size, stripe_count of 2 and
stripe_size of 2 MiB; the first half of the file will be filled with 'A's
and the second half will be filled with 'B's:

               0      4M     8M       Obj1     Obj2
               +------+------+       +----+   +----+
        file:  | AAAA | BBBB |       | AA |   | AA |
               +------+------+       |----|   |----|
                                     | BB |   | BB |
                                     +----+   +----+

If we copy_file_range this file into a new file (which needs to have the
same file layout!), then it will start by copying the object starting at
file offset 0 (Obj1).  And then it will copy the object starting at file
offset 4M -- which is Obj1 again.

Unfortunately, the solution for this is to not allow remote object copies
to be performed when the file layout stripe_count is not 1 and simply
fallback to the default (VFS) copy_file_range implementation.

Cc: stable@vger.kernel.org
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>