fs: dcache: Use READ_ONCE when accessing i_dir_seq

fs: dcache: Use READ_ONCE when accessing i_dir_seq

[ Upstream commit 8cc07c808c9d595e81cbe5aad419b7769eb2e5c9 ]

i_dir_seq is subject to concurrent modification by a cmpxchg or
store-release operation, so ensure that the relaxed access in
d_alloc_parallel uses READ_ONCE.

Reported-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>