fuse: check attributes staleness on fuse_iget()

Function fuse_direntplus_link() might call fuse_iget() to initialize a new
fuse_inode and change its attributes. If fi->attr_version is always
initialized with 0, even if the attributes returned by the FUSE_READDIR
request is staled, as the new fi->attr_version is 0, fuse_change_attributes
will still set the staled attributes to inode. This wrong behaviour may
cause file size inconsistency even when there is no changes from
server-side.

To reproduce the issue, consider the following 2 programs (A and B) are
running concurrently,

        A                                               B
----------------------------------      --------------------------------
{ /fusemnt/dir/f is a file path in a fuse mount, the size of f is 0. }

readdir(/fusemnt/dir) start
//Daemon set size 0 to f direntry
                                        fallocate(f, 1024)
                                        stat(f) // B see size 1024
                                        echo 2 > /proc/sys/vm/drop_caches
readdir(/fusemnt/dir) reply to kernel
Kernel set 0 to the I_NEW inode

                                        stat(f) // B see size 0

In the above case, only program B is modifying the file size, however, B
observes file size changing between the 2 'readonly' stat() calls. To fix
this issue, we should make sure readdirplus still follows the rule of
attr_version staleness checking even if the fi->attr_version is lost due to
inode eviction.

To identify this situation, the new fc->evict_ctr is used to record whether
the eviction of inodes occurs during the readdirplus request processing.
If it does, the result of readdirplus may be inaccurate; otherwise, the
result of readdirplus can be trusted. Although this may still lead to
incorrect invalidation, considering the relatively low frequency of
evict occurrences, it should be acceptable.

Link: https://lore.kernel.org/lkml/[email protected]/
Link: https://lore.kernel.org/lkml/[email protected]/

Reported-by: Jiachen Zhang <[email protected]>
Suggested-by: Miklos Szeredi <[email protected]>
Signed-off-by: Zhang Tianci <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>

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