pmbootstrap install: set inodes count dynamically

While !2552 (merged) fixes #2568 (closed) and #2242 (closed), it creates another bug during installation

(023151) [23:23:53] (native) % mkfs.ext4 -O ^metadata_csum -F -q -L pmOS_root -N 1000000 /dev/installp2
mkfs.ext4: inode_size (256) * inodes_count (1000000) too big for a
	filesystem with 248832 blocks, specify higher inode_ratio (-i)
	or lower inode count (-N).

This problem can be solved by checking how many inodes rootfs chroot has and then assign a bigger number that doesn't exceed the filesystem limit.

E.g. rootfs_inodes*2 rounded to 2^N*1024 inodes