bpf: Remove the WARN from arena_get_unmapped_area()

The user range of an arena must not cross a 4 GiB boundary. Thus
arena_get_unmapped_area() asks the mm for 2 * len bytes and rounds the
result up if necessary. If user_vm_start is not 0, addr must be equal to
it. A WARN_ON_ONCE() assumes that the mm returns addr. But addr is only a
hint without MAP_FIXED, and the mm can return a different range that
crosses a boundary. This is not a kernel bug.

Remove the WARN_ON_ONCE() and its -EFAULT. arena_map_mmap() already
compares vm_start with user_vm_start under arena->lock. If the rounded
result is user_vm_start, mmap() succeeds. If not, it returns -EBUSY.

Fixes: 317460317a02 ("bpf: Introduce bpf_arena.")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261101190535.1454146-7-alexei.starovoitov@gmail.com

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