os/bluestore/BlueFS: fix device_migrate_to_* to handle varying alloc sizes

os/bluestore/BlueFS: fix device_migrate_to_* to handle varying alloc sizes

The previous implementation moved extents individually.  This caused
problems when moving an extent with a small alloc_size that wasn't
a multiple of the target device's alloc_size.

Instead, identify files with extents that need to be moved, and then read
the file in its entirety and rewrite it in its entirety.

Signed-off-by: Sage Weil <sage@redhat.com>