ksmbd: avoid zeroing the read buffer in smb2_read()

ksmbd: avoid zeroing the read buffer in smb2_read()

smb2_read() allocates the read payload buffer with kvzalloc(), zeroing up
to max_read_size bytes (1MB or more with multichannel) on every read,
only to immediately overwrite the region with file data via kernel_read().

The zero-fill is pure overhead: ksmbd_vfs_read() returns the number of
bytes actually read ('nbytes'), and only those nbytes are ever consumed -
they are pinned into the response iov (ksmbd_iov_pin_rsp_read()), sent
over the RDMA channel (smb2_read_rdma_channel()), or copied by the
compression path (ksmbd_compress_response() uses iov_len == nbytes). The
ALIGN(length, 8) tail padding and any short-read remainder are never read
or transmitted, so they need not be initialized.

Use kvmalloc() instead to skip the redundant zeroing. This reduces CPU
and memory-bandwidth usage on large sequential reads.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>