netfs: Fix missing xas_retry() calls in xarray iteration

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

netfs: Fix missing xas_retry() calls in xarray iteration

netfslib has a number of places in which it performs iteration of an xarray

whilst being under the RCU read lock. It *should* call xas_retry() as the

first thing inside of the loop and do "continue" if it returns true in case

the xarray walker passed out a special value indicating that the walk needs

to be redone from the root[*].

Fix this by adding the missing retry checks.

[*] I wonder if this should be done inside xas_find(), xas_next_node() and

suchlike, but I'm told that's not an simple change to effect.

This can cause an oops like that below. Note the faulting address - this

is an internal value (|0x2) returned from xarray.

BUG: kernel NULL pointer dereference, address: 0000000000000402

...

RIP: 0010:netfs_rreq_unlock+0xef/0x380 [netfs]

...

Call Trace:

netfs_rreq_assess+0xa6/0x240 [netfs]

netfs_readpage+0x173/0x3b0 [netfs]

? init_wait_var_entry+0x50/0x50

filemap_read_page+0x33/0xf0

filemap_get_pages+0x2f2/0x3f0

filemap_read+0xaa/0x320

? do_filp_open+0xb2/0x150

? rmqueue+0x3be/0xe10

ceph_read_iter+0x1fe/0x680 [ceph]

? new_sync_read+0x115/0x1a0

new_sync_read+0x115/0x1a0

vfs_read+0xf3/0x180

ksys_read+0x5f/0xe0

do_syscall_64+0x38/0x90

entry_SYSCALL_64_after_hwframe+0x44/0xae

Changes:

========

ver #2)

- Changed an unsigned int to a size_t to reduce the likelihood of an

overflow as per Willy's suggestion.

- Added an additional patch to fix the maths.