linux-user: Modify 'target_to_host/host_to_target_itimerspec()'

Functions 'target_to_host_itimerspec()' and 'host_to_target_itimerspec()'
are used to convert values of type 'struct itimerspec' between target and
host. This type has 'struct timespec' as its fields. That is the reason
why this patch introduces a little modification to the converting functions
to be implemented using already existing functions that convert 'struct timespec':
'target_to_host_timespec()' and 'host_to_target_timespec()'. This makes the
code of 'target_to_host_itimerspec()' and 'host_to_target_itimerspec()' more
clean and readable.

Signed-off-by: Filip Bozuta <Filip.Bozuta@syrmia.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200722153421.295411-2-Filip.Bozuta@syrmia.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>