ext4fs: fix "invalid extent block" error

For files where we actually have extent indexes following
an extent header (ext_block->eh_depth != 0), the do/while
loop from ext4fs_get_extent_block() does not select the
proper extent index structure.

For example, if we have:

ext_block->eh_depth = 1
ext_block->eh_entries = 1
fileblock = 0
index[0].ei_block = 0

the do/while loop will exit with i set to 0 and the
ext4fs_get_extent_block() function will return 0, even if
there was a valid extent index structure following the
header.

Signed-off-by: Ionut Nicu <ioan.nicu.ext@nsn.com>
Signed-off-by: Mathias Rulf <mathias.rulf@nsn.com>