Low [CVE-2026-64158] Fix write streaming disablement if fd open O_RDWR

In the Linux kernel, the following vulnerability has been resolved: netfs: Fix write streaming disablement if fd open O_RDWR In netfs_perform_write(), "write streaming" (the caching of dirty data in dirty but!uptodate folios) is performed to avoid the need to read data that is just going to get immediately overwritten.

However, this is/will be disabled in three circumstances: if the fd is open O_RDWR, if fscache is in use (as we need to round out the blocks for DIO) or if content encryption is enabled (again for rounding out purposes).

The idea behind disabling it if the fd is open O_RDWR is that we'd need to flush the write-streaming page before we could read the data, particularly through mmap. But netfs now fills in the gaps if ->read_folio() is called on the page, so that is unnecessary.

Further, this doesn't actually work if a separate fd is open for reading. Fix this by removing the check for O_RDWR, thereby allowing streaming writes even when we might read.

This caused a number of problems with the generic/522 xfstest, but those are now fixed. The write streaming feature, which caches dirty data to avoid unnecessary reads, was incorrectly disabled when a file descriptor was opened with `O_RDWR` (read-write) mode.