In the former commits, the callback of isochronous context runs on work
process, thus no need to use atomic memory allocation.

This commit replaces GFP_ATOMIC with GCP_KERNEL in the callback for user
client.

Tested-by: Edmund Raile <edmund.raile@protonmail.com>
Link: https://lore.kernel.org/r/20240904125155.461886-5-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>