mark_buffer_dirty semantical change in 2.3.43

[Andrea Arcangeli]

In 2.3.43 I changed mark_buffer_dirty() to be blocking as in 2.2.x to fix
all the filesystems out there in 2.3.43/fs/*/*.[ch].

I am providing a new call __mark_buffer_dirty() not blocking usable inside
critical sections that holds fs spinlocks. Using __mark_buffer_dirty()  
means that you'll have to call balance_dirty() as soon as you can sleep as
you was just used to do with the mark_buffer_dirty() of 2.3.x kernels
previous than 2.3.43.

Andrea