md/bitmap: fix GPF in write_page caused by resize race

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

md/bitmap: fix GPF in write_page caused by resize race

A General Protection Fault occurs in write_page() during array resize:

RIP: 0010:write_page+0x22b/0x3c0 [md_mod]

This is a use-after-free race between bitmap_daemon_work() and

__bitmap_resize(). The daemon iterates over `bitmap->storage.filemap`

without locking, while the resize path frees that storage via

md_bitmap_file_unmap(). `quiesce()` does not stop the md thread,

allowing concurrent access to freed pages.

Fix by holding `mddev->bitmap_info.mutex` during the bitmap update.