ocfs2: fix use-after-free in ocfs2_fault() when VM_FAULT_RETRY

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

ocfs2: fix use-after-free in ocfs2_fault() when VM_FAULT_RETRY

filemap_fault() may drop the mmap_lock before returning VM_FAULT_RETRY,

as documented in mm/filemap.c:

"If our return value has VM_FAULT_RETRY set, it's because the mmap_lock

may be dropped before doing I/O or by lock_folio_maybe_drop_mmap()."

When this happens, a concurrent munmap() can call remove_vma() and free

the vm_area_struct via RCU. The saved 'vma' pointer in ocfs2_fault() then

becomes a dangling pointer, and the subsequent trace_ocfs2_fault() call

dereferences it -- a use-after-free.

Fix this by saving ip_blkno as a plain integer before calling

filemap_fault(), and removing vma from the trace event. Since

ip_blkno is copied by value before the lock can be dropped, it

remains valid regardless of what happens to the vma or inode

afterward.