Possible bug in pmfs_increase_btree_height

[paulwedeck]

Steps to reproduce:

• Allocate all available memory of a WineFS partition to one file (fallocate -l <large number> <myfile>). I tried 100000000 for a 16MB partition.
• Now create a new file and allocate 4097 bytes to it (fallocate -l 4097 <second file>)

This should lead to the fallocate process getting killed and this error message in the kernel log "BUG: unable to handle kernel NULL pointer dereference at 0000000000000000".

I think the issue is that pmfs_increase_btree_height fails due to lack of memory but returns 0 (no error) anyways.
The subsequent allocation attempt tries to read from the inner tree node which is a null block which results in this error.