callout: fix using shared rmlocks

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()).

Reviewed by: markj
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D34959

(cherry picked from commit a879e40ca2a9e95b3e3dc4810127d3cf105ec0d3)