bpf: dispatch to sleepable file dynptr

bpf: dispatch to sleepable file dynptr

File dynptr reads may sleep when the requested folios are not in
the page cache. To avoid sleeping in non-sleepable contexts while still
supporting valid sleepable use, given that dynptrs are non-sleepable by
default, enable sleeping only when bpf_dynptr_from_file() is invoked
from a sleepable context.

This change:
  * Introduces a sleepable constructor: bpf_dynptr_from_file_sleepable()
  * Override non-sleepable constructor with sleepable if it's always
  called in sleepable context

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20251026203853.135105-10-mykyta.yatsenko5@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>