ARM: perf: Check that current->mm is alive before getting user callchain

An event may occur when an mm is already released.

As per commit 20afc60f
 'x86, perf: Check that current->mm is alive before getting user callchain'

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Jean Pihet <jean.pihet@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>