fs/dcache: Use read_word_at_a_time() in dentry_string_cmp()

fs/dcache: Use read_word_at_a_time() in dentry_string_cmp()

dentry_string_cmp() performs the word-at-a-time reads from 'cs' and may
read slightly more than it was requested in kmallac().  Normally this
would make KASAN to report out-of-bounds access, but this was
workarounded by commit df4c0e36f1b1 ("fs: dcache: manually unpoison
dname after allocation to shut up kasan's reports").

This workaround is not perfect, since it allows out-of-bounds access to
dentry's name for all the code, not just in dentry_string_cmp().

So it would be better to use read_word_at_a_time() instead and revert
commit df4c0e36f1b1.

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>