Honor CONFIG_FS_POSIX_ACL kernel option

The required Posix ACL interfaces are only available for kernels
with CONFIG_FS_POSIX_ACL defined. Therefore, only enable Posix
ACL support for these kernels. All major distribution kernels
enable CONFIG_FS_POSIX_ACL by default.

If your kernel does not support Posix ACLs the following warning
will be printed at ZFS module load time.

"ZFS: Posix ACLs disabled by kernel"

Signed-off-by: Massimo Maggi <me@massimo-maggi.eu>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1825