reiserfs: check kstrdup failure

Check out-of-memory failure of the kstrdup option. Note that the argument
"arg" may be NULL (in that case kstrup returns NULL), so out of memory
condition happened if arg was non-NULL and kstrdup returned NULL.

The patch also changes the call to replace_mount_options - if we didn't
pass any filesystem-specific options, we don't call replace_mount_options
(thus we don't erase existing reported options).

Note that to properly report options after remount, the reiserfs
filesystem should implement the show_options method. Without the
show_options method, options changed with remount replace existing
options.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>