linux-user: allow NULL arguments to mount

Calls to the mount syscall can legitimately provide NULL as the value
for the source of filesystemtype arguments, which QEMU would previously
reject & return -EFAULT to the target program. An example of this is
remounting an already mounted filesystem with different properties.

Instead of rejecting such syscalls with -EFAULT, pass NULL along to the
kernel as the target program expects.

Additionally this patch fixes a potential memory leak when DEBUG_REMAP
is enabled and lock_user_string fails on the target or filesystemtype
arguments but a prior argument was non-NULL and already locked.

Since the patch already touched most lines of the TARGET_NR_mount case,
it fixes the indentation & coding style for good measure.

Signed-off-by: Paul Burton <paul@archlinuxmips.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
