ecryptfs: string copy cleanup

Clean up overcomplicated string copy, which also gets rid of this
bogus warning:

fs/ecryptfs/main.c: In function 'ecryptfs_parse_options':
include/asm/arch/string_32.h:75: warning: array subscript is above array bounds

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>