e2fsck: propagate number of threads

e2fsck: propagate number of threads

Sometimes, such as in orphan_inode case, e2fsck_pass1
is called after reading the block bitmaps. This results in
reading the block bitmap sequentially and multithreading
only gets kicked in later. Fix the thread count earlier
while setting up the file system.

E2fsprogs-commit: 3f56676f4daa7379dbd868a4114910f20d49527d

Change-Id: I51ab36ad8094c605a32e4b7929fadb326d77169e
Signed-off-by: Saranya Muruganandam <saranyamohan@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>