Remove unused MMAP functions

The following functions were required for the OpenSolaris mmap
implementation. Because the Linux VFS does most the most heavy
lifting for us they are not required and are being removed to
keep the code clean and easy to understand.

• zfs_null_putapage()
• zfs_frlock()
• zfs_no_putpage()

Signed-off-by: Brian Behlendorf <behlendorf@llnl.gov>