15b1eb142c changed the callout code to store the CALLOUT_SHAREDLOCK flag
in c_iflags (where it used to be c_flags), but failed to update the
check in softclock_call_cc(). This resulted in the callout code always
taking the write lock, even if a read lock had been requested (with
the CALLOUT_SHAREDLOCK flag in callout_init_rm()).
MFC after: 1 week
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