bcachefs: make nocow lock waits SRCU-aware

[Komzpa]

Summary

• threads the active btree_trans into blocking nocow lock waits
• uses the existing trans_wait_event() wrapper so long waits can drop SRCU via the current short-wait budget
• leaves the fast trylock path unchanged and avoids the blanket bch2_trans_unlock_long() replacement from the conflicted kernel PR

Context

This targets the nocow-write stall path reported in koverstreet/bcachefs#680, where writeback was blocked around bch2_nocow_write() / nocow locking while the transaction still held SRCU. It is also the narrow form of the maintainer direction on koverstreet/bcachefs#1069: start with the normal unlock path, but escalate long blocking waits through the transaction-aware wait helper instead of dropping SRCU unconditionally at every callsite.

Tests

• bcachefs: cover nocow SRCU wait pressure ktest#88 adds focused nocow write pressure coverage for this SRCU-held wait path.

Validation

• git diff --check
• make generate_version && BINDGEN=/home/kom/.cargo/bin/bindgen LIBCLANG_PATH=/usr/lib/llvm-18/lib CARGO_BUILD_JOBS=4 make -j4 bcachefs
• safe staged kbuild module compile without unload/install side effects:
  make -C /lib/modules/$(uname -r)/build M=/home/kom/tmp/bcachefs-module-nocow-srcu-wait modules -j4
• GitHub Actions: Nix Flake actions run 28427644776, 15/15 jobs passed on head 896612eb72c2741ce554d147a65006475adfc042

Related: koverstreet/bcachefs#1069

[koverstreet]

Merged — cherry-picked onto master (36c7a31, authorship preserved). The SRCU-aware nocow waits are the right fix; the write.c caller was blocking under a held SRCU read lock.

Followed it with a small cleanup commit on our side: since bch2_data_update_init() now takes @trans and the nocow path only runs with ctxt != NULL (promote path bails earlier), the ctxt ? ctxt->trans : NULL and if (ctxt) guards were dead — @trans == ctxt->trans there, so we pass @trans directly. Also corrected the stale 'we don't have a pointer to the trans' comment: we do now, but promotes intentionally don't block on nocow locks so they can't pile up. Thanks!