parser: remove unnecessary strlen()

No functional change.  Cache strlen() result to avoid recalculating it up
to 3 times on the worst case.

Reduces code size a little by 32 bytes:
   text    data     bss     dec     hex filename
   1385       0       0    1385     569 lib/parser.o-BEFORE
   1353       0       0    1353     549 lib/parser.o-AFTER

Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>