Handle cas failure when the compare succeeds

When locking a priority inherit mutex we perform a compare and swap
operation to try and acquire the mutex. This may fail even when the
compare succeeds.

Check and handle this case.

PR: 263825
Reviewed by: kib, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35150

(cherry picked from commit 11a6ecd4258b9108fb19420ec5db297f6d99a842)