xfs_healer: fix getmntent race in weakhandle

Codex points out that getmntent() can't be used in threaded programs
because it might employ hidden global static buffers.  GNU and musl libc
provide a getmntent_r variant that requires the caller to establish the
buffers, so let's use that.

Cc: linux-xfs@vger.kernel.org # v7.0.0
Fixes: 54c0ba68b007b3 ("xfs_healer: use getmntent to find moved filesystems")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>