NFS: replace __get_free_page() with kmalloc() in nfs_show_devname()

NFS: replace __get_free_page() with kmalloc() in nfs_show_devname()

nfs_show_devname() allocates a tmemporary buffer __get_free_page().

kmalloc() is a better API for such use and it also provides better
scalability and more debugging possibilities.

Replace use of __get_free_page() with kmalloc().

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Link: https://patch.msgid.link/20260523-b4-fs-v1-5-275e36a83f0e@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>