e2fsck: factor out code to clear an inode into e2fsck_clear_inode()

e2fsck: factor out code to clear an inode into e2fsck_clear_inode()

Factor out code to clear a bogus inode and update e2fsck's internal
data structures accordingly into a common routine,
e2fsck_clear_inode().  This saves about 200 bytes in the compiled x86
e2fsck executable, and makes the code more maintainable in the
long-term.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>