xfs: fix delalloc write failures in software-provided atomic writes

With the 20 Oct 2025 release of fstests, generic/521 fails for me on
regular (aka non-block-atomic-writes) storage:

QA output created by 521
dowrite: write: Input/output error
LOG DUMP (8553 total operations):
1( 1 mod 256): SKIPPED (no operation)
2( 2 mod 256): WRITE 0x7e000 thru 0x8dfff (0x10000 bytes) HOLE
3( 3 mod 256): READ 0x69000 thru 0x79fff (0x11000 bytes)
4( 4 mod 256): FALLOC 0x53c38 thru 0x5e853 (0xac1b bytes) INTERIOR
5( 5 mod 256): COPY 0x55000 thru 0x59fff (0x5000 bytes) to 0x25000 thru 0x29fff
6( 6 mod 256): WRITE 0x74000 thru 0x88fff (0x15000 bytes)
7( 7 mod 256): ZERO 0xedb1 thru 0x11693 (0x28e3 bytes)

with a warning in dmesg from iomap about XFS trying to give it a
delalloc mapping for a directio write. Fix the software atomic write
iomap_begin code to convert the reservation into a written mapping.
This doesn't fix the data corruption problems reported by generic/760,
but it's a start.

Cc: stable@vger.kernel.org # v6.16
Fixes: bd1d2c21d5d249 ("xfs: add xfs_atomic_write_cow_iomap_begin()")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>