vmalloc: rename temporary variable in __insert_vmap_area()

Rename redundant 'tmp' to fix following sparse warnings:

 mm/vmalloc.c:296:34: warning: symbol 'tmp' shadows an earlier one
 mm/vmalloc.c:293:24: originally declared here

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>