LINUX: Build fixes for older kernels

LINUX: Build fixes for older kernels

Error and warning fixes for older kernels in osi_compat.h:

 - In afs_posix_test_lock, remove the assignment in the conditional to
   silence a warning

 - Call getsockopt for kernel_getsockopt, instead of setsockopt

 - Missing end brace in afs_try_to_freeze

 - Prototype find_exported_dentry, since Linux doesn't give us one

Change-Id: Iae56bb0b0405bfd23dfd68a22c1d7922663b1442
Reviewed-on: http://gerrit.openafs.org/2946
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>