nfsd: fetch direct I/O alignment for files handed to the filecache

nfsd_file_do_acquire() fetches a file's direct I/O alignment
attributes (nf_dio_mem_align, nf_dio_offset_align and
nf_dio_read_offset_align) only on the branch where NFSD opens the
file itself. The other branch, taken when the caller supplies an
already-open struct file, stores that file in the nfsd_file and
leaves the three alignment fields zero.

NFSv4 OPEN with CREATE takes the supplied-file branch:
nfsd4_create_file() creates the file with dentry_create() and passes
the resulting struct file as open->op_filp, which nfs4_get_vfs_file()
hands to nfsd_file_acquire_opened(). The cached nfsd_file therefore
keeps zero alignment for its whole life, and
nfsd_write_dio_iters_init() refuses direct I/O for every WRITE to a
file the client has just created.

Observed on an export whose backing device reports a dio_mem_align of
4: all 16 of 16 1 MiB NFSv4.2 writes that followed an OPEN(CREATE)
took the buffered path, while 16 of 16 writes to the same file
reopened without CREATE went direct.

Hoist the nfsd_file_get_dio_attrs() call out of the open branch so
that it runs once, for both branches, whenever the acquire has
otherwise succeeded. A getattr failure now fails the acquire on the
supplied-file branch exactly as it already does on the open branch.

Fixes: d11f6cd1bb ("NFSD: filecache: add STATX_DIOALIGN and STATX_DIO_READ_ALIGN support")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-fable-5.1
Signed-off-by: David Flynn <david.flynn@hammerspace.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Link: https://patch.msgid.link/20260908163448.30841-4-snitzer@kernel.org
Signed-off-by: Chuck Lever <cel@kernel.org>

fs/nfsd/filecache.c

@@ -1286,9 +1286,9 @@ open_file:
 			}
 			status = nfserrno(ret);
 			trace_nfsd_file_open(nf, status);
-			if (status == nfs_ok)
-				status = nfsd_file_get_dio_attrs(fhp, nf);
 		}
+		if (status == nfs_ok)
+			status = nfsd_file_get_dio_attrs(fhp, nf);
 	} else
 		status = nfserr_jukebox;
 	/*