bcachefs: bch2_get_random_u64_below()

steal the (clever) algorithm from get_random_u32_below()

this fixes a bug where we were passing roundup_pow_of_two() a 64 bit
number - we're squaring device latencies now:

[  +1.681698] ------------[ cut here ]------------
[  +0.000010] UBSAN: shift-out-of-bounds in ./include/linux/log2.h:57:13
[  +0.000011] shift exponent 64 is too large for 64-bit type 'long unsigned int'
[  +0.000011] CPU: 1 UID: 0 PID: 196 Comm: kworker/u32:13 Not tainted 6.14.0-rc6-dave+ #10
[  +0.000012] Hardware name: ASUS System Product Name/PRIME B460I-PLUS, BIOS 1301 07/13/2021
[  +0.000005] Workqueue: events_unbound __bch2_read_endio [bcachefs]
[  +0.000354] Call Trace:
[  +0.000005]  <TASK>
[  +0.000007]  dump_stack_lvl+0x5d/0x80
[  +0.000018]  ubsan_epilogue+0x5/0x30
[  +0.000008]  __ubsan_handle_shift_out_of_bounds.cold+0x61/0xe6
[  +0.000011]  bch2_rand_range.cold+0x17/0x20 [bcachefs]
[  +0.000231]  bch2_bkey_pick_read_device+0x547/0x920 [bcachefs]
[  +0.000229]  __bch2_read_extent+0x1e4/0x18e0 [bcachefs]
[  +0.000241]  ? bch2_btree_iter_peek_slot+0x3df/0x800 [bcachefs]
[  +0.000180]  ? bch2_read_retry_nodecode+0x270/0x330 [bcachefs]
[  +0.000230]  bch2_read_retry_nodecode+0x270/0x330 [bcachefs]
[  +0.000230]  bch2_rbio_retry+0x1fa/0x600 [bcachefs]
[  +0.000224]  ? bch2_printbuf_make_room+0x71/0xb0 [bcachefs]
[  +0.000243]  ? bch2_read_csum_err+0x4a4/0x610 [bcachefs]
[  +0.000278]  bch2_read_csum_err+0x4a4/0x610 [bcachefs]
[  +0.000227]  ? __bch2_read_endio+0x58b/0x870 [bcachefs]
[  +0.000220]  __bch2_read_endio+0x58b/0x870 [bcachefs]
[  +0.000268]  ? try_to_wake_up+0x31c/0x7f0
[  +0.000011]  ? process_one_work+0x176/0x330
[  +0.000008]  process_one_work+0x176/0x330
[  +0.000008]  worker_thread+0x252/0x390
[  +0.000008]  ? __pfx_worker_thread+0x10/0x10
[  +0.000006]  kthread+0xec/0x230
[  +0.000011]  ? __pfx_kthread+0x10/0x10
[  +0.000009]  ret_from_fork+0x31/0x50
[  +0.000009]  ? __pfx_kthread+0x10/0x10
[  +0.000008]  ret_from_fork_asm+0x1a/0x30
[  +0.000012]  </TASK>
[  +0.000046] ---[ end trace ]---

Reported-by: Roland Vet <vet.roland@protonmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>