ext4: do not convert to indirect with bigalloc enabled

With bigalloc feature enabled we do not support indirect addressing at all
so we have to prevent extent addressing to indirect addressing
conversion in this case. The problem has been introduced with the commit
"ext4: support simple conversion of extent-mapped inodes to use i_blocks"

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>