checkpatch: improve warning message for "needless if" case

Add an 'and' to the sentence so that it looks better:

  WARNING: debugfs_remove(NULL) is safe and this check is probably not required

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>