quota: allocate dquot_hash with kmalloc()

quota: allocate dquot_hash with kmalloc()

dquot_init() allocates a single page for dquot_hash with
__get_free_pages().

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

Replace use of __get_free_pages() with kmalloc() and get rid of the order
variable that remained 0 for more than 20 years.

Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Link: https://patch.msgid.link/20260523-b4-fs-v1-1-275e36a83f0e@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>