# DF-3150 — `HAMMER_MREC_TYPE_REC_NODATA` case is dead code; NODATA records are installed without data

## What

`hammer_ioc_mirror_write()` dispatches on
`mrec.head.type & HAMMER_MRECF_TYPE_MASK` = `0x800000FF`
(hammer_mirror.c:429, hammer_ioctl.h:383), but the NODATA flag lives in bit
`0x10000000` (hammer_ioctl.h:388). The case label
`HAMMER_MREC_TYPE_REC_NODATA` (`0x10000001`) can therefore never match — the
masked value of a NODATA record is plain `1` (`HAMMER_MREC_TYPE_REC`) — so
NODATA records take the full install path of `hammer_ioc_mirror_write_rec`
instead of being skipped, and the record is created with `data_len == 0`
(a hole) even though the master has data for it.

The read side genuinely emits such records: hammer_mirror.c:244-248,272 sets
`type = HAMMER_MREC_TYPE_REC | HAMMER_MRECF_NODATA` whenever the user asked
for `HAMMER_IOC_MIRROR_NODATA` (histogram generation).

## Impact

Silent data loss / integrity corruption on a mirror target fed a
NODATA-flagged stream (malicious or accidental). No kernel memory-safety
effect — hence severity Low.

## Reproduce (runtime proof, stock kernel)

```
# fresh 2GB hammer fs on vn0, file /mnt/f1 (see README of DF-3148)
/tmp/mirror_poc /mnt/f1 nodata      # NODATA-flagged REC mrec
/tmp/mirror_poc /mnt/f1 readback    # HAMMERIOC_MIRROR_READ the range
```

Expected (buggy) result — the record IS created:
```
MIRROR_WRITE rc=0 head.error=0 head.flags=00000000 count=80   <- processed
MIRROR_READ rc=0 error=0 count=664
  mrec[0] ... obj=1234 key=100 ctid=1000 dtid=0 btype=52 rt=0012 ... dlen=0
```
Had the `case HAMMER_MREC_TYPE_REC_NODATA:` skip path been reachable, no
obj=1234 record would exist and readback would show only the PFS root records.

## Fix

`fix.diff` (shared with DF-3148/3149): include `HAMMER_MRECF_NODATA` in the
switch mask so the existing skip case becomes reachable. Validated on a
patched kernel: NODATA mrec consumed (count advances), readback shows no
obj=1234 record.