Replace zf_rwlock with a mutex

The rwlock implementation on linux does not perform as well as mutexes.
We can realize a performance benefit by replacing the zf_rwlock with a
mutex.  Local microbenchmarks show ~50% improvement, and over NFS we see
~5% improvement on several of the ZFS Performance Tests cases,
especially randwrite and seq_write.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Tony Nguyen <[email protected]>
Reviewed-by: Olaf Faaland <[email protected]>
Signed-off-by: Matthew Ahrens <[email protected]>
Closes #9062 

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