ceph: avoid fs reclaim while using current->journal_info

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

ceph: avoid fs reclaim while using current->journal_info

handle_reply() stores a `ceph_mds_request` pointer in
`current->journal_info` while filling the inode and dentry cache from
an MDS reply.

An allocation in this section can enter direct reclaim and prune
dentries from another filesystem. If this dirties an ext4 inode, ext4
starts a JBD2 transaction. JBD2 interprets the Ceph request in
`current->journal_info` as a journal handle and dereferences the
request's `r_tid` as `h_transaction`, causing a kernel crash, e.g.:

Unable to handle kernel paging request at virtual address 00000000077b4818
[...]
Internal error: Oops: 0000000096000004 [#1] SMP
Modules linked in:
CPU: 6 UID: 0 PID: 2699135 Comm: kworker/6:3 Tainted: G W 6.18.38-i3 #1113 NONE
[...]
Workqueue: ceph-msgr ceph_con_workfn
pstate: 80400009 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : jbd2__journal_start+0x2c/0x208
lr : __ext4_journal_start_sb+0x100/0x178
[...]
Call trace:
jbd2__journal_start+0x2c/0x208 (P)
__ext4_journal_start_sb+0x100/0x178
ext4_dirty_inode+0x3c/0x90
__mark_inode_dirty+0x58/0x400
iput.part.0+0x2b0/0x370
iput+0x18/0x30
dentry_unlink_inode+0xc0/0x158
__dentry_kill+0x80/0x250
shrink_dentry_list+0x90/0x130
prune_dcache_sb+0x60/0x98
super_cache_scan+0xe8/0x190
do_shrink_slab+0x174/0x388
shrink_slab+0xd8/0x4c0
shrink_node+0x31c/0x908
do_try_to_free_pages+0xd0/0x508
try_to_free_pages+0x11c/0x238
__alloc_frozen_pages_noprof+0x4d0/0xdd0
__folio_alloc_noprof+0x18/0x70
__filemap_get_folio+0x248/0x440
ceph_readdir_prepopulate+0x570/0x9e8
mds_dispatch+0x1424/0x1ba0
ceph_con_process_message+0x74/0xa0
ceph_con_v1_try_read+0x3a0/0x1510
ceph_con_workfn+0x260/0x460

Enter a scoped NOFS allocation context and leave it after clearing
`journal_info`. This prevents filesystem reclaim from recursing into
another filesystem while the field contains Ceph-private data.