mds: dump SnapRealm for src/dest in link operations

mds: dump SnapRealm for src/dest in link operations

So we can verify the state of the SnapRealm if we hit EXDEV.

Now looks like:

    2026-01-29T12:05:52.603-0500 7defd1a64640 20 mds.0.server src_realm snaprealm(0x10000000002 seq 1 lc 0 cr 1 cps 2 snaps={} subvol last_modified 0.000000 change_attr 0 is_snapdir_visible 1 0x5c1a558a66c0)
    2026-01-29T12:05:52.603-0500 7defd1a64640 20 mds.0.server dest_realm snaprealm(0x10000000002 seq 1 lc 0 cr 1 cps 2 snaps={} subvol last_modified 0.000000 change_attr 0 is_snapdir_visible 1 0x5c1a558a66c0)

Signed-off-by: Patrick Donnelly <pdonnell@ibm.com>
Fixes: https://tracker.ceph.com/issues/74652