arm64: ptrace: Remove redundant overrun check from compat_vfp_set()

compat_vfp_set() checks for userspace trying to write an excessive
amount of data to the regset.  However this check is conspicuous
for its absence from every other _set() in the arm64 ptrace
implementation.  In fact, the core ptrace_regset() already clamps
userspace's iov_len to the regset size before the individual regset
.{get,set}() methods get called.

This patch removes the redundant check.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>