pagecache: report page-writeback errors from sync() by return value

[gburd]

Summary

pagecache::sync() threw a C++ error when a page writeback (cached_page_write::writeback()) failed. Its only caller is the munmap / msync path: vfs_file::sync(), invoked from mmu::file_vma::sync(), which already converts the outcome to a return value and surfaces any error through its own sys_fsync() tail. The fsync(2) syscall uses a separate VOP_FSYNC path and never relied on this throw, so no caller depended on the exception.

Throwing from the munmap path is both unnecessary and fragile: raising an exception during munmap() can abort when the C++ unwinder cannot complete in the calling context, turning a recoverable page-writeback error into a fatal abort() that takes down the whole instance.

Change

• pagecache::sync() returns int (0 on success, or the writeback errno) instead of throwing.
• vfs_file::sync() consumes the return value; mmu::file_vma::sync() continues to report the error via sys_fsync().
• The declaration in include/osv/pagecache.hh is updated to match.

Behavior is unchanged on success. On a writeback error, sync() now returns the errno rather than throwing, and the page is left marked dirty for retry exactly as before.

[gburd]

Added a companion commit: the base file::sync() (the virtual default in include/osv/file.h) also threw make_error(ENOSYS) and is reached from the same munmap page-writeback path (mmu::file_vma::sync -> _file->sync()). Unmapping a file-backed mapping whose file type does not override sync() therefore raised an exception during munmap() that cannot be unwound from that path and bottoms out in abort(). A file type with no cached writable pages has nothing to flush, so the default is now a no-op. Together with the pagecache change this removes all C++ throws from the munmap writeback path.