vfs: read file_handle only once in handle_to_path.

We used to read file_handle twice.  Once to get the amount of extra
bytes, and once to fetch the entire structure.

This may be problematic since we do size verifications only after the
first read, so if the number of extra bytes changes in userspace between
the first and second calls, we'll have an incoherent view of
file_handle.

Instead, read the constant size once, and copy that over to the final
structure without having to re-read it again.

Git-commit: 161f873b89136eb1e69477c847d5a5033239d9ba
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git



Change-Id: Id64c2a7b7e751b0d55c5bbe871152a4b0f88c630
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Zhoulu Luo <zluo@codeaurora.org>