uafs: avoid type-punning in get_user_struct()

uafs: avoid type-punning in get_user_struct()

There's no need for type-punning here; usr_getspecific() is a macro
that just assigns to the variable whose address we provide, so the
cast was just unnecessary (and erroneous) obfuscation.  This is the
only caller of usr_getspecific(), so if it needs to be more complex
in the future, it should probably just be open-coded here.

Change-Id: I81286cbce9a00908aa87073b728b07d8bcc8d1d1
Reviewed-on: http://gerrit.openafs.org/7799
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>