block: Name the RQF flags enum

block: Name the RQF flags enum

Commit 5f89154e8e9e3445f9b59 ("block: Use enum to define RQF_x bit
indexes") converted the RQF flags to an anonymous enum, which was
a beneficial change. This patch goes one step further by naming the enum
as "rqf_flags".

This naming enables exporting these flags to BPF clients, eliminating
the need to duplicate these flags in BPF code. Instead, BPF clients can
now access the same kernel-side values through CO:RE (Compile Once, Run
Everywhere), as shown in this example:

    rqf_stats = bpf_core_enum_value(enum rqf_flags, __RQF_STATS)

Suggested-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20250306-rqf_flags-v1-1-bbd64918b406@debian.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>