logfs: check for the return value after calling find_or_create_page()

In get_mapping_page(), after calling find_or_create_page(), the return
value should be checked.

 This patch has been provided:
http://www.spinics.net/lists/linux-fsdevel/msg66948.html

 but not been
applied now.

Signed-off-by: Younger Liu <liuyiyang@hisense.com>
Cc: Younger Liu <younger.liucn@gmail.com>
Cc: Vyacheslav Dubeyko <slava@dubeyko.com>
Reviewed-by: Prasad Joshi <prasadjoshi.linux@gmail.com>
Cc: Jörn Engel <joern@logfs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>