CVE-2026-98907: btrfs: protect sb_write_pointer() with invalidate lock

In the Linux kernel, the following vulnerability has been resolved:

btrfs: protect sbwritepointer() with invalidate lock

sbwritepointer() reads the super block from the block device page cache using readcachepagegfp(). This has the same race with BLKBSZSET as the one fixed by commit 3f29d661e568 ("btrfs: sync read disk super and set block size").

Take the mapping invalidate lock around readcachepagegfp() to serialize the read against block size changes.