ext4: mballoc: avoid garbage value from err

ext4: mballoc: avoid garbage value from err

clang's static analysis warning: fs/ext4/mballoc.c
line 4178, column 6, Branch condition evaluates to a garbage value.

err is uninitialized and will be judged when 'len <= 0' or
it first enters the loop while the condition "!ext4_sb_block_valid()"
is true. Although this can't make problems now, it's better to
correct it.

Signed-off-by: Su Hui <suhui@nfschina.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20230725043310.1227621-1-suhui@nfschina.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>